Specific alteration of the form of selenium in fetal liver on maternal methylmercury treatment

We have recently reported that maternal administration of methylmercury caused a striking increase in the selenium concentration in fetal liver accompanied by a decrease in selenium-dependent glutathione peroxidase (GSH-Px) activity. These changes resulted in the lowered bioavailability of selenium as far as the GSH-Px activity was concerned. The present study demonstrated that maternal administration of methylmercury caused a specific alteration of the form of selenium in fetal liver. Sephadex G-200 gel filtration of liver cytosols revealed an additional major peak of selenium in the fetal livers of mice treated with methylmercury. This peak was not present in the liver, kidney, or placenta of mothers treated with methylmercury.     

Methyl mercury degradation in mink

Degradation of methyl mercury by mink was investigated in a series of experiments. Mink were fed daily with a diet containing methyl mercury-contaminated fish. Contents of total mercury, methyl mercury and selenium were determined in different tissues from the animals, as were the contents in faces. Of the total amount of mercury detected, only about 73% was found as methyl mercury. In liver and kindney the proportions were and 55%, respectively. Selenium contents were low compared with those found in marine mammals where equivalent amounts of mercury and selenium on a molar basis been reported. The conditions in mink are compared with earlier studies on cats and marine mammals.     

Effect of various amounts of selenium on the metabolism of mercuric chloride in mice

Male ddY mice were given one injection of (1) mercury (mercuric chloride) simultaneously with various doses of selenium (sodium selenite), (2) mercury alone, or (3) various doses of selenium alone. The interaction between mercury and selenium in the liver and kidneys at 1, 5, 24, 120, and 240 hr after administration was investigated. The concentrations of mercury in the liver of mice receiving mercury and selenium simultaneously were higher than those after administration of mercury alone, while the concentrations of mercury in the kidney decreased markedly over a 1-120 hr period after administration, depending on the dose of selenium administered simultaneously with mercury. Clearly, selenium had a different effect on the accumulation of mercury in the liver and kidneys. Subcellular distribution studies revealed that mercury and selenium which were administered simultaneously were incorporated into the crude nuclear and mitochondrial fractions as stable complexes. The transport of these complexes to the kidneys seems to be limited. In addition, gel filtration of supernatant fractions of liver and kidney through a Sephadex G-75 column indicated that the proportion of mercury bound to metallothionein fraction decreased depending on the dose of selenium administered simultaneously with the mercury. This reduction was attributed to the decreased synthesis of mercury-thionein due to a reduction in the activity of Hg2+ which results from binding between mercury and selenium in the cells.     

In vitro enhancement of erythrocyte merucry uptake by spironolactone.

Four groups of male Wistar rats were fed the following regimen for 40 days: (1) 20 ppm methylmercury chloride (MMC); (2) 20 ppm MMC + 3 ppm sodium selenite; (3) 3 ppm sodium selenite; (4) basal diet. The basal diet which contained 0.4 ppm “organic selenium” originating mainly from fish meal and wheat was resumed on day 41. Protective effect of selenite over toxicity of methylmercury was observed in terms of both growth rate and morbidity.Concentrations of total mercury, methylmercury and selenium were determined on Days 0, 20, 41, 47, 54, and 61 in the brain, liver, kidney, and blood. It was noted that methylmercury increased accumulation of selenium in all the organs analyzed while selenium retention varied according to the type of selenium and the organs. Modification by selenite, despite its protective effect, remained equivocal in regard to the organ accumulation of mercury and its retention therein.     

Release of mercury from dental amalgam fillings in pregnant rats and distribution of mercury in maternal and fetal tissues

Mercury vapor released from a single amalgam restoration in pregnant rats and mercury concentrations in maternal and fetal rat tissues were studied. Dental treatment was given on day 2 of pregnancy. Mercury concentration in air sample drawn from the metabolism chamber with the rat was measured serially for 24 h on days 2, 8 and 15 of pregnancy. An average mercury concentration in the air samples from the rats given amalgam restorations was 678.6+/-167.5 ng/day on day 2. The average mercury concentration in the air samples tended to decline as time elapsed but a marked amount (423.2+/-121.5 ng/day) was observed even on day 15. The amount of mercury in the air samples increased 7--20-fold after chewing. The placement of the single amalgam restoration (3.8--5.5 mg in weight) increased the levels of mercury approximately three to 6 times in the maternal brain, liver, lung, placenta and 20 times in the kidneys. The highest mercury concentration among fetal organs was found in the liver, followed by the kidneys and brain. Mercury concentrations in maternal organs and fetal liver were significantly higher than those of the controls, and concentrations in maternal whole blood, erythrocytes and plasma, and in fetal whole blood were also significantly higher. Mercury concentrations in the fetal brain, liver, kidneys and whole blood were lower than those of the maternal tissues.     

Differential changes of glutathione S-transferase activity by dietary selenium

Dietary selenium deficiency produced increased activity of the glutathione S-transferases in the liver, kidney and duodenal mucosa. In these tissues, the residual activity of total glutathione peroxidase that included selenium-independent activity was considerably higher than that of selenium-dependent glutathione peroxidase. The enhanced activity of glutathione S-transferases was restored to control level 48 hr after an injection of selenite equivalent to the amount of daily selenium intake. Under the same conditions, selenium-dependent glutathione peroxidase activity increased with time and reached 11.9, 11.6 and 46.2% of the activity in the liver, kidney and duodenal mucosa of selenium-supplemented rats, respectively, 48 hr after selenite injection, whereas total glutathione peroxidase activity was not altered except in the kidney. These differential changes of glutathione S-transferase activity were intimately related to those of selenium-dependent glutathione peroxidase activity produced by selenium depletion and repletion, suggesting that the glutathione S-transferase activity was regulated by dietary selenium. Present findings support the idea that glutathione S-transferases having selenium-independent glutathione peroxidase activity function as a substitute for selenium-dependent glutathione peroxidase in selenium-deficient rats.     

Mechanism of hepatocellular damage in rat caused by low serum selenium

The aim of this paper is to investigate the mechanism of hepatocellular damage in rats caused by low serum selenium. Thirty six rats were randomly divided into 2 groups: group A (fed with low-selenium diet from the Keshan Disease area with the content of selenium being 0.017 mg/kg); group B [fed with selenium-supplemented diet and 0.3 mg/L selenium (Na₂SeO₃) was added to the drinking water]. Both were respectively fed for 12 weeks. At the end of the 12^th week, the levels of serum selenium, glutathione peroxidase (GPX) and malondialdehyde (MDA) in hepatic tissue were measured; the hepatocellular ultrastructure and apoptosis were observed as well. The levels of serum selenium and GPX in group A were markedly lower than those in group B. MDA level in group A was significantly higher than that in group B. Under the electron microscope (EM), the mitochondria were remarkably changed in group A. The rate of liver cell apoptosis appeared much higher in group A as well. It indicated that the damage caused by selenium deficiency was through the process of oxidation. Selenium deficiency led to apoptosis of hepatocytes where oxidative damage to mitochondria might be the cause.     

Selenium and drug metabolism--I. Multiple modulations of mouse liver enzymes

Male albino mice were raised on diets containing less than 10 ppb selenium (Se-) or supplemented with 0.5 ppm selenium (Se+) for 6 months. In the (Se-) group total liver selenium was less than 10% of the control, liver selenium-dependent glutathione peroxidase (GSH-Px) less than 2%. The specific activities of catalase and superoxide dismutase showed essentially no differences between the dietary groups. Several phase I-related specific enzyme activities were measured in liver microsomes. No significant differences between the two animal groups were found for cytochrome P-450 and b 5 content, NADH-cytochrome b 5 reductase, as well as for aniline hydroxylation and aminopyrine dealkylation rates. In (Se-) microsomes, NADPH-cytochrome P-450 reductase activity was about half that found in (Se+) microsomes. An increase in microsomes from (Se-) mice was found for 7-ethoxycoumarine deethylation rate (460%), cytochrome P-450 hydroperoxidase activity (170%), and heme oxygenase (276%). The N-oxidation rate of the flavin-containing monooxygenase decreased by 35%, the N-demethylation rate by 50% in (Se-) animals. Stopped-flow measurements of the reduction rates of microsomal pigments did not support evidence for limitations in microsomal electron supply during selenium deficiency. Among the phase II reactions examined, sulfotransferase activity towards 4-nitrophenol was 47% of the controls in Se-deficient liver cytosols while UDP-glucuronyl transferase activity towards this substrate increased to 215%. Glutathione-S-transferase activity was much higher in (Se-) livers than in (Se+): 310% with 1,2-dichloro-4-nitrobenzene, 255% with 1-chloro-2,4-dinitrobenzene and 120% with ethacrynic acid as substrate. The data indicate that in addition to GSH-Px many other enzyme activities in mouse liver are affected by prolonged dietary selenium deficiency. These effects might be useful in assessing the severity of selenium deficiency. A microsomal selenium-dependent metabolic modulator is discussed as a possible mechanism.     

The effect of selenium on the biliary excretion and organ distribution of mercury in the rat after exposure to methyl mercuric chloride.

The influence of selenium compounds on the biliary excretion and the organ distribution of mercury after injection of methyl mercuric chloride (4 mumol/kg) have been tested. Selenite, seleno-di-N-acetylglycine and seleno-methionine strongly inhibited the biliary excretion of mercury. Selenite even in a molar dose of 1/40 of the methyl mercury dose inhibited the biliary excretion of mercury. The less toxic seleno-di-N-acetylglycine was needed in larger molar doses and did not act as rapidly as selenite. Biliary excreted methyl mercury is known to be partly reabsorbed in the gut. Subsequently a part of it is deposited in the kidneys since drainage of the bile lowered the kidney content of mercury. Rats given selenium compounds in combination with bile drainage showed further reduction of the kidney mercury content than bile duct drainage alone. Thus the demonstrated lowering effect of selenium compounds on the kidney mercury content cannot be completely explained by an inhibition of biliary excretion of mercury. The mercury concentration in the brain was increased by the selenium compounds; the effect being dependent of the selenium dose reaching a maximum at an equimolar selenite--to methyl mercury dose ratio. The mechanisms by which selenium influences the methyl mercury kinetics are discussed.     

Selenium and drug metabolism--II. Independence of glutathione peroxidase and reversibility of hepatic enzyme modulations in deficient mice

Male mice were fed a diet containing less than 0.01 ppm selenium (Se-) for 6 months. A control group received the same diet containing 0.5 ppm selenium (Se+). In the livers of the Se- animals a drastic decrease in glutathione peroxidase (GSH-Px) activity was observed. It reached undetectable levels after 17 days of the Se- diet. At that time, GSH-transferase activity began to increase significantly, followed by changes in many other enzyme activities. After the 60th day, these enzyme modulations had reached a plateau with the following percentage changes compared to controls: GSH-transferases: 320% (1,2-dichloro-4-nitrobenzene), 218% (1-chloro-2,4-dinitrobenzene); glutathione reductase: 160%; ethoxycoumarin deethylase: 330%; cytochrome P-450-hydroperoxidase: 230%; heme oxygenase: 240%; UDP-glucuronyltransferase: 200%; GSH-thioltransferase: 64%; sulphotransferase: 62%; NADPH-cytochrome-P-450-reductase: 65%; flavin-containing mono-oxygenase: 57%. No significant changes were observed for GSH-transferase activity assayed with ethacrynic acid or for microsomal H2O2 formation and aniline hydroxylase activity. In single-pulse repletion experiments by injection of 250 micrograms selenium/kg body wt, different individual time constants for the recovery process of the enzymatic perturbations were observed. The half-times for the recovery ranged from 5.7 hr for the microsomal NADPH-cytochrome-P-450 reductase to over 29 hr for GSH-Px up to 44 hr for part of the GSH-transferase activity. 250 micrograms selenium/kg body wt were needed to restore 50% of GSH-Px activity in the long-term Se- mice compared to Se+ controls. All other enzymatic changes in the Se- mice needed a dose of 7 micrograms selenium/kg body wt for 50% restorage . The results demonstrate that processes other than those related to GSH-Px take place in a later phase of selenium deficiency in mouse liver with a chronologically common beginning. The different repletion and depletion kinetics as well as the different need of these processes for the trace element are discussed with respect to the existence of two separate selenium pools.     

Effect of maternal selenium deficiency on the teratogenicity of methylmercury

The teratogenicity of methylmercury was examined in selenium-deficient and selenium-sufficient mice of the ICR strain. A single oral dose of 75 mumol/kg of methylmercuric chloride (MMC) on day 10, 11 or 12 of gestation caused similar incidences of a cleft palate in the selenium-deficient and selenium-sufficient mice. The treatment on day 11 led to the highest incidence of a cleft palate in both groups. These results suggest that a maternal selenium deficiency has no effect on the incidence or on the critical gestational period for an MMC-induced cleft palate.     

Selenium and drug metabolism--III. Relation of glutathione-peroxidase and other hepatic enzyme modulations to dietary supplements

Male mice were fed a torula yeast-based diet containing different amounts of added selenium for a period of 4 months. Liver glutathione peroxidase activity assayed with H2O2 showed a logarithmic dependence on dietary selenium with a saturation plateau above 2 ppm Se and an extrapolated zero of 0.02 ppm Se. In contrast, liver selenium content and GSH-Peroxidase activity showed a linear correlation. Glutathione peroxidase activity became undetectable at a liver Se content of about 90 ng Se/g liver wet wt. Thus, about 10% of liver selenium is not related to GSH-Px activity. Five dietary groups were supplemented, respectively, with 0, 0.05, 0.5, 5.0 and 10 ppm Se in the form of Na2SeO3. Some changes in drug metabolism enzymes were observed with the high Se diets. An increase occurred in Non-Se-GSH activity as well as in ethacrynic acid-assayed GSH transferase, these are interpreted as early signs of Se toxicity. The diet containing 0.01 ppm Se with no supplementary Se produced the multiple hepatic enzyme modulations which were previously reported. The animals raised on this very low Se diet had normal hepatic contents of glutathione, alpha-tocopherol, calcium, magnesium, iron, zinc, copper and manganese compared to controls supplemented with 0.5 ppm Se. However, significant changes in the microsomal fatty acid pattern were observed while the total phospholipid content as well as membrane fluidity showed no differences between the two dietary groups.     

Effect of cystine,selenium, and fish protein on the toxicity and metabolism of methylmercury in rats

Investigations were made of the effect of 0.4% cystine, 0.6 ppm selenium, and fish protein on the toxicity and metabolism of mercury from methylmercuric chloride in male and female weanling rats. When diets with 10% protein and adequate amounts of other nutrients contained 25 ppm mercury, cystine reduced toxicity, selenium as sodium selenite had a greater effect in reducing toxicity, and the combination of cystine and selenium produced a considerably greater additive effect, as measured by increased growth and survival time. Toxicity signs were reduced when fish protein replaced casein in the basal diet, and a 20% protein level from either source reduced toxicity symptoms as compared to a 10% protein level. Cystine, selenium, and fish protein did not inhibit mercury toxicity by increasing its elimination from the body via the urine and feces and, in fact, slightly more mercury was retained when cystine and selenium were added to diets. Few differences were found in the deposition of mercury in tissues that could be attributed to cystine, selenium, and fish protein. The major difference found was in one experiment in which mercury concentrations in the kidney were considerably lower when diets contained cystine, selenium, and fish protein. Possible explanations for the effects observed in these studies included a reduction in mercury accumulation and, hence, damage in the kidney, complexing of mercury by the additives evaluated, and conversion of organic mercury to the less toxic inorganic form.     

Organ clearance of 75SeO3(2-) and 203HgCl2 administered separately and simultaneously to mice

A study on organ clearance of mercury (²⁰³HgCl₂) and selenium (⁷⁵SeO₃^2?) given separately or simultaneously in single doses by intraperitoneal injections to mice has been performed. The simultaneous administrations are handled in molar ratios (Hg/Se) less, than equal to or greater than one. Liver, kidneys, spleen and blood contain most of the mercury and selenium, administered, while heart, lungs, skin, muscle and brain only contain small amounts. Both elements are retained to a higher degree in especially kidneys, liver, spleen and blood when co-administered than when administered alone. In kidneys, clearance rate of selenium is found to be independent of mercury administration, with an effective halflife 11.2–13.5 days in accordance with the whole elimination. The clearance of mercury is, however, strongly dependent on coadministered selenium. At low molar ration (Hg/Se ? 1) of administered doses clearance rate is identical to that of selenium, compared to an effective halflife when administered alone. In liver the effective halflife of selenium when administered alone or coadministered with mercury (Hg/Se ratio < 1) is found identical to that of kidneys: 11.2–12.5 days. At increasing molar ratios retention of selenium increases and clearance rate decreases. A marked decrease in mercury clearance from liver is induced by simultaneously administered selenium. The effective halflife becoming extremely administered selenium. The effective halflife becoming extremely long and under certain conditions even an accumulation is found to take place. Concordant relations are found for liver and spleen.Selenium in blood is only affected to a minor degree by mercury. Clearance rate for mercury is decreased by selenium. At a molar ratio between doses (Hg/Se) ? 1 clearance rate is approximately identical to that of selenium. It is indicated that selenium metabolism is quantitatively rather than qualitatively influenced by mercury as mercury administration provokes a higher retention of a given dose of selenium while clearance rate in kidneys and blood is only influenced by high molar ratios (Hg/Se). Contrary to selenium, mercury metabolism is altered both qualitatively and quantitatively as retention as well as clearance rate are influenced by selenium in all organs and in all molar ratios given in kidneys and blood. It is assumed that at least 2 mechanisms exist in the metabolic pathways of mercury: (1) binding to metallothionein; and (2) binding to selenium containing metabolic compounds. As selenium induced an increase in mercury retention it is questioned whether selenium is beneficial in case of chroniic exposure to inorganic mercury. It also implies the question of bioavailability of selenium in animal food items.     

An explanation for strain and sex differences in renal uptake of methylmercury in mice.

The present investigation was designed to elucidate the mechanism for strain and sex differences in renal methylmercury accumulation, in five mouse strains, viz. BALB/cA, C57BL/6N, CBA/JN, C3H/HeN and ICR. Strain and sex comparisons of factors which influence renal mercury accumulation were made. Strain and sex differences were observed in renal mercury accumulation 4 h after methylmercuric chloride (MMC) (1 mumol/kg, s.c.) injection. Glutathione (GSH) content in liver and kidney showed significant strain and sex differences. Pretreatment with 1,2-dichloro-4-nitrobenzene (DCNB), to deplete hepatic GSH without affecting renal non-protein thiol (NPSH) level, led to a dose-dependent decrease in hepatic and plasma GSH concentrations that correlated with decreased mercury levels in the kidney 10 min after MMC (1 mumol/kg, i.v.) injection. This indicates that hepatic and plasma GSH levels are related to mercury accumulation into the kidney. Renal gamma-glutamyltranspeptidase (gamma-GTP) activity significantly varied among the strains, and in BALB/cA and ICR, renal gamma-GTP activity in males was about 2-fold higher than that in females. Renal gamma-GTP activity was also correlated with the renal mercury content. These results suggest that strain and sex differences in renal accumulation of mercury are attributable to differences in tissue GSH content and possibly to differences in renal gamma-GTP activity.     

Effects of cadmium treatment on selenium-dependent and selenium independent glutathione peroxidase activities and lipid peroxidation in the kidney and liver of rats maintained on various levels of dietary selenium

Rats fed a basal, low-selenium diet, or this diet supplemented with 0.1 ppm and 1.0 ppm selenium and treated with cadmium, showed significant reductions in the activity of the selenoenzyme glutathione peroxidase in kidney and liver. Cadmium treatment resulted in a significant increase in the activity of selenium-independent glutathione peroxidase activity in the liver of selenium-supplemented rats. Selenium-independent glutathion peroxidase activity was significantly reduced in the kidney of rats fed the basal low-selenium diet. There was no significant increase in lipid peroxidation in any of the groups studied. Cadmium concentrations in the kidney and liver of these animals ranged from about 250 to 700 g Cd/g tissue, dry weight.Supported by NIEHS Center Grant ES-00159 and a Grant from the Selenium-Tellurium Development Association     

Inhibition of cholinesterase enzymes following a single dermal dose of chlorpyrifos and methyl parathion, alone and in combination, in pregnant rats

Pregnant Sprague-Dawley rats (14-18 d of gestation) were treated with either a single dermal subclinical dose of 30 mg/kg (15% of dermal LD50) chlorpyrifos (O,O-diethyl-O-[3,5,6-trichloro-2-pyridinyl] phosphorothioate) or a single dermal subclinical dose of 10 mg/kg (15% of dermal LD50) methyl parathion (O,O-dimethyl O-4-nitrophenyl phosphorothioate) or the two in combination. Chlorpyrifos inhibited maternal and fetal brain acetylcholinesterase (AChE) activity within 24 h of dosing, (48% and 67% of control activity, respectively). Following application of methyl parathion, peak inhibition of maternal and fetal brain AChE activity occurred at 48 h and 24 h after dosing (17% and 48% of control activity, respectively). A combination of chlorpyrifos and methyl parathion produced peak inhibition of maternal and fetal brain AChE activity at 24 h postdosing (35% and 73% of control activity, respectively). Maternal and fetal brain AChE activity recovered to various degrees of percentage of control 96 h after dosing. Application of methyl parathion or chlorpyrifos alone or in combination significantly inhibited maternal plasma butyrylcholinesterase (BuChE) activity. No significant inhibition of fetal plasma BuChE activity was detected. Peak inhibition of maternal liver BuChE occurred 24 h after application of methyl parathion or chlorpyrifos alone or in combination (64%, 80%, and 61% of control activity, respectively). Significant inhibition of placental AChE occurred within 24 h after application of methyl parathion or chlorpyrifos alone or in combination. The results suggest that methyl parathion and chlorpyrifos, alone or in combination, were rapidly distributed in maternal and fetal tissues, resulting in rapid inhibition of cholinesterase enzyme activities. The lower inhibitory effect of the combination could be due to competition between chlorpyrifos and methyl parathion for cytochrome P-450 enzymes, resulting in inhibition of the formation of the potent cholinesterase inhibitor oxon forms. The faster recovery of fetal plasma BuChE is attributed to the de novo synthesis of cholinesterase by fetal tissues compared to maternal tissues.     

A novel biologically active seleno-organic compound--II. Activity of PZ 51 in relation to glutathione peroxidase

The anti-inflammatory compound 2-phenyl-1,2-benzoisoselenazol-3(2H)-on (PZ 51) catalysed GSSG formation from GSH in the presence of hydroperoxides in an NADPH/GSSG reductase system with the following rates (delta log GSH/min per molar selenium): 1.1 X 10(6) with H2O2, 1.2 X 10(6) with butylhydroperoxide, 1.7 X 10(6) with cumenehydroperoxide. The reaction catalysed by the sulphur analogue of PZ 51 was negligible. Similar results were obtained in a direct assay of GSH-Px activity based on GSH estimation by dithionitrobenzoate. The activation energy of the reaction was determined as 55 kJ/mol . deg in the presence of 30 mumol/1 PZ 51 compared to 36.5 kJ/mol . deg obtained in the presence of 1 nmol/1 pure GSH-Px isolated from bovine red blood cells. In mouse liver microsomes, NADPH-dependent aminopyrine dealkylation was totally inhibited in the presence of 50 mumol/1 PZ 51. In vivo experiments with Se-deficient mice showed that the Se-moiety of PZ 51 is not available for the synthesis of the selenoenzyme GSH-Px after dietary treatment or i.p. doses up to 25 mg Se as PZ 51 per kg body wt. After oral administration of labelled PZ 51, unlike with selenite, no radioactivity was incorporated into GSH-Px within 48 hr. The data suggest that several similarities between PZ 51 and the active site of GSH-Px exist, resulting in the capability of the compound to catalyse the GSH-Px reaction. An extracellular pharmacodynamic action of the drug seems likely.     

Effects of ethanol on methyl mercury toxicity in rats

This study was designed to investigate the effect of different doses of ethanol on the morbidity, mortality, and distribution of mercury in the tissues of groups of rats treated orally once daily with methyl mercury chloride (MMC: 5 mg/kg . d) for 10 consecutive days. Ethanol potentiated the toxicity of methyl mercury in terms of neurological manifestations (hindleg crossings and abnormal gait) and mortality. The magnitude of effect depended on the concentration of ethanol administered. The concentration of mercury in the kidney and brain also increased with the dose of ethanol given. These findings indicate that epidemiologic studies designed to evaluate methyl mercury toxicity must take into account the multiple environmental burdens that can affect the population cumulatively and simultaneously.     

Arsenicals in maternal and fetal mouse tissues after gestational exposure to arsenite

Exposure of pregnant C3H/HeNCR mice to 42.5- or 85-ppm of arsenic as sodium arsenite in drinking water between days 8 and 18 of gestation markedly increases tumor incidence in their offspring. In the work reported here, distribution of inorganic arsenic and its metabolites, methyl arsenic and dimethyl arsenic, were determined in maternal and fetal tissues collected on gestational day 18 of these exposure regimens. Tissues were collected from three females and from associated fetuses exposed to each dosage level. Concentrations of total speciated arsenic (sum of inorganic, methyl, and dimethyl arsenic) were higher in maternal tissues than in placenta and fetal tissues; total speciated arsenic concentration in placenta exceeded those in fetal tissues. Significant dosage-dependent (42.5 ppm versus 85 ppm of arsenite in drinking water) differences were found in total speciated arsenic concentrations in maternal lung (p<0.01) and liver (p<0.001). Total speciated arsenic concentrations did not differ significantly between dosage levels for maternal blood or for fetal lung, liver, and blood, or for placenta. Percentages of inorganic, methyl, or dimethyl arsenic in maternal or fetal tissues were not dosage-dependent. Over the range of total speciated arsenic concentrations in most maternal and fetal tissues, dimethyl arsenic was the most abundant arsenical. However, in maternal liver at the highest total speciated arsenic concentration, inorganic arsenic was the most abundant arsenical, suggesting that a high tissue burden of arsenic affected formation or retention of methylated species in this organ. Tissue concentration-dependent processes could affect kinetics of transfer of inorganic arsenic or its metabolites from mother to fetus.