Toxicity of acenaphthene and isophorone to early life stages of fathead minnows

Flow-through 96-hr and early-life-stage toxicity tests were conducted with acenaphthene and isophorone, using fathead minnows (Pimephalespromelas) as test animals. The 96-hr LC₅₀'s were 608g/L for acenaphthene and 145 and 255 mg/L for isophorone, depending on fish age. No-effect concentrations from early-life-stage exposures were 413g acenaphthene/L and 14 mg isophorone/L; these showed good agreement with published toxicity data.     

Toxicity of zinc and bleached kraft mill effluent to larval english sole (Parophrys vetulus) and topsmelt (Atherinops affinis)

Toxicity test protocols were developed for larvae of two species of marine fishes of the Pacific coast of the United States. During winter months, larvae were obtained from adult English sole (Parophrys vetulus) spawned in the laboratory. Newly-hatched, nonfeeding larvae were used in 96-hr experiments to determine their acute sensitivity to inorganic and organic toxicants. Exposure to ZnSO₄ produced a 96-hr LC₅₀ of 14.5 mg Zn/L. A complex and variable organic effluent (untreated bleached kraft mill effluent; BKME) yielded 96-hr LC₅₀s ranging from 1.2–9.3% BKME.Larvae were obtained from eggs collected from wild and laboratory populations of the spring and summer-spawning topsmelt (Atherinops affinis). Both acute (96-hr) mortality and longer-term (7–14 days) growth responses of topsmelt larvae to BKME were determined. Acute toxicity levels (96-hr LC₅₀s) ranged from 6.2–9.7% BKME. No Observed Effect Levels (NOELs) for 7 and 14 day exposures ranged from 1.0–3.0% BKME.     

Acute and embryo-larval toxicity of phenolic compounds to aquatic biota

Because of the prevalence of phenolic compounds in various types of effluents, both acute and embryo-larval bioassays were performed on eight phenolic compounds with rainbow trout, fathead minnows andDaphnia pulicaria. In flow-through bioassays, the 96-hr LC₅₀ values for rainbow trout and fathead minnows ranged from <0.1 mg/L for hydroquinone to >100 mg/L for resorcinol.Daphnia pulicaria was consistently the least sensitive species tested as measured in 48-hr bioassays, while fathead minnows and rainbow trout varied in their relative sensitivity to phenolics as measured in 96-hr tests. Fathead minnows were more sensitive to phenol at 25°C than at 14°C.In embryo-larval bioassays with phenol, fathead minnow growth was significantly reduced by 2.5 mg/L phenol, while rainbow trout growth was significantly reduced by 0.20 mg/L phenol. For both species the embryolarval effects concentration was 1.1% of the 96-hr LC₅₀. Another embryolarval bioassay was attempted withp-benzoquinone, a highly toxic phenolic compound found in fossil fuel processing wastewaters, which was discontinued because the compound was rapidly degraded chemically or biologically in the headtank and aquaria.Work funded under an Interagency Agreement between the U.S. Department of Energy and the U.S. Environmental Protection Agency under Contract No. DE-AS20-79 LC 01761 to the Rocky Mountain Institute of Energy and Environment, University of Wyoming.     

Effects of naphthalene and benzene on fathead minnows and rainbow trout

Fathead minnows (FHM) and rainbow trout (RBT) were used in flow-through bioassays to determine the acute toxicity of benzene and naphthalene, and to determine the embryo-larval effects of naphthalene on FHM. On an acute basis, naphthalene was more toxic than benzene (naphthalene LC₅₀ values were 1.6 mg/L for RBT and 7.9 mg/L for FHM; benzene LC₅₀ values were 5.3 mg/L for RBT and >15.1 mg/L for FHM). In the embryo-larval test naphthalene significantly (= 0.05) reduced FHM growth at concentrations as low as 0.85 mg/L. The highest concentration producing no effect was 0.45 mg/L naphthalene, which was 5.7% of the FHM 96-hr LC₅₀. Based upon long-term no-effects naphthalene concentration, the best estimate of the maximum acceptable toxicant concentration (MATC) was >0.45 to <0.85 mg/L naphthalene.Work funded under an Interagency Agreement between the U.S. Department of Energy and the U.S. Environmental Protection Agency under Contract No. DE-AS20-79 LC 01761 to the Rocky Mountain Institute of Energy and Environment, University of Wyoming     

Changes in brain dopamine levels,oocyte growth and spermatogenesis in rainbow trout,Oncorhynchus mykiss,following sublethal cyanide exposure

Whole brain dopamine (DA) and norepinephrine (NE) levels were measured in sexually maturing (2 years+) male and female rainbow trout,Oncorhynchus mykiss, following posure to 0.01 mg/L hydrogen cyanide (HCN). Following a 12 day exposure period in July and August 1988, whole brain DA levels of HCN exposed fish were significantly higher (p < 0.05), relative to control fish, as measured by high performance liquid chromatography (HPLC). Brain NE levels were unaffected by HCN exposure. Whole brain DA and NE levels showed a strong correlation in control fish (r=+0.81), but not in HCN exposed fish (r=+0.28), likely due to altered DA levels in the latter group. No significant differences were found in brain DA and NE levels between males and females. Mean diameters of oocyte from ovaries of the vitellogenic females were significantly (p < 0.01) reduced from 226 to 183 m in control and HCN exposed fish respectively. Testes from males revealed significantly (p < 0.001) higher numbers of spermatogonial cysts in HCN exposed fish. Evidence is given that chronic exposure to sublethal levels of HCN significantly alters brain DA levels in both sexes of rainbow trout, reduces growth in vitellogenic oocytes of the ovary in females and interferes with the passage of spermatogonia to the spermatocyte stage in sexually maturing males. Collectively, these results suggest that sublethal HCN affects the reproductive mechanisms via the hypophyseal-gonad axis in sexually maturing rainbow trout.     

Acute toxicity of butylbenzyl phthalate to shiner perch (Cymatogaster aggregata)

The Clean Water Act of 1971 directed the Environmental Protection Agency to establish ambient water quality criteria for several classes of elements and compounds, including phthalate esters. Multispecies acute toxicity data are required to develop these criteria. Static bioassay LC₅₀s for butylbenzyl phthalate (BBP) for a single species of marine fish based on nominal doses range from 3 mg/L to 440 mg/L. Flow-through bioassays were used in this study of BBP with shiner perch (Cymatogaster aggregata). Using measured exposure concentrations, 96-hr LC₅₀s averaged 0.51 mg/L. Effects on schooling behavior were found at 0.08 mg/L and coloration at 0.24 mg/L. Coupled with the behavioral changes, reduced brain levels of epinephrine found in surviving fish indicated that the mode of acute toxicity for BBP may be through its effects on the catecholamines of the central adrenergic nervous system.     

Toxicity of underground coal gasification condenser water and selected constituents to aquatic biota

The acute and embryo-larval toxicity of the Laramie Energy Technology Center's Hanna-3 underground coal gasification (UCG) condenser water and its constituents were studied in continuous-flow bioassays. The 96-hr LC₅₀ dilution values for untreated Hanna-3 UCG condenser water were 0.1% for rainbow trout, 0.11% for fathead minnows and the 48-hr LC₅₀ dilution forDaphnia pulicaria was 0.18%. Separate 96-hr acute tests with phenol, ammonia, and ammonia plus phenol showed that these two constituents, acting synergistically, were the major constituents affecting the acute toxicity of this coal conversion effluent to fishDaphnia pulicaria, on the other hand, was relatively insensitive to phenol exposure; the primary constituent of Hanna-3 UCG condenser water affecting this species was ammonia.A previously described model was used for predicting the toxicity of effluents with high concentrations of phenol and ammonia to confirm our hypothesis that the acute toxicity of Hanna-3 UCG condenser water to fish was primarily due to the presence of phenol and ammonia. Using the Hanna-3 concentrations of phenol and ammonia in this formula, it was calculated that the 96-hr LC₅₀ values for rainbow trout and fathead minnows exposed to Hanna-3 condenser water would be 0.11% and 0.28%, respectively; values which are near the observed acute toxicity of Hanna-3 condenser water.In a 30-day embryo-larval exposure, fathead minnow egg hatchability, growth, and survival were significantly reduced at 0.04%, 0.02% and 0.01% Hanna-3 condenser water, respectively. At a Hanna-3 dilution of 0.01%, the phenol and un-ionized ammonia concentrations were calculated to be 0.23 mg/L and 0.14 mg/L, respectively. The phenol and un-ionized ammonia concentrations are within ranges expected to produce the long-term effects which were observed.Work funded under an Interagency Agreement between the U.S. Department of Energy and the U.S. Environmental Protection Agency under Contract No. DE-AS20-79 LC 01761 to the Rocky Mountain Institute of Energy and Environment, University of Wyoming.     

Short-term lethality and sediment avoidance assays with endrin-contaminated sediment and two oligochaetes from Lake Michigan

Mean 96-hr LC₅₀ values and standard deviations for the oligochaetesS. heringianus andL. hoffmeisteri exposed to endrin-contaminated sediment were 2,588±1,974 g/g dry weight sediment for 4 assays and 2,725±955 g/g for 2 assays, respectively. Mixed species testing data suggested that the toxicity toL. hoffmeisteri was reduced in the presence ofS. heringianus, yet further testing is required. Ninety-six hour EC₅₀ burrowing avoidance values for both species (19 and 15.3 g/g forS. heringianus and 59 g/g forL. hoffmeisteri) were approximately 46 and 150 times lower than their respective mean 96-hr LC₅₀ values. Both S.heringianus andL. hoffmeisteri initially burrowed in contaminated sediment and then returned to the surface in numbers somewhat proportional to the sediment concentration and the length of exposure. Future use of oligochaete behavioral responses to subiethal sediment contamination for pollutant impact on benthic communities is promising.     

Toxicity of iodine,iodide, and iodate to Daphnia magna and rainbow trout (Oncorhynchus mykiss)

The acute toxicity (96-h LC₅₀) of aqueous stable iodine species (I^–, IO ₃ ^– , I₂) to rainbow trout and Daphnia magna were measured at three individual concentrations of hardness, total organic carbon, and chloride. Rainbow trout were most sensitive to I₂ (LC₅₀0.53 mg/L), and much less sensitive to IO ₃ ^– (LC₅₀220 mg/L) or I^– (LC₅₀860 mg/L). Daphnia magna were equally sensitive to I₂ (LC₅₀0.16 mg/L) and I^– (LC₅₀0.17 mg/L), but were less sensitive to IO ₃ ^– (LC₅₀10.3 mg/L). The external and internal radiological dose imparted by equivalent molar quantities of radioactive ¹²⁵I, ¹²⁹I, and ¹³¹I were calculated for both the Daphnia and trout using the LC₅₀ values obtained from a standard water treatment. As expected, the dose from ¹²⁵I and ¹³¹I would exceed the expected lethal dose rate long before a chemically toxic level is reached. In contrast, a molar concentration of ¹²⁹I likely to cause death by chemical toxicity would impart a radiological dose less than that expected to be lethal. Thus, for short-lived aquatic organisms, risks due to chemical toxicity of ¹²⁹I may exceed risks due to its radioactive emissions.     

Acute toxicities and hematological effects of two substituted naphthoquinones in channel catfish

The acute toxicities and hematological effects of menadione (2-methyl-1,4-naphtho-quinone) and dichlone (2,3-dichloro-1,4-naphtho-quinone) were examined in yearling channel catfish (Ictalurus punctatus). These compounds served as models for quinones, which comprise a diverse group of compounds that enter aquatic systems from numerous sources. The 96-hr LC₅₀'s measured were 720 g/L and 42 g/L for menadione and dichlone, respectively. In subsequent experiments, fish were exposed for 21 days to concentrations of 0, 0.25, 0.50, and 0.75 times the LC₅₀ value of each compound. For both compounds, generally dose-dependent increases in methemoglobin concentrations and dose-dependent reductions in hemoglobin concentrations and hematocrit ratios were observed. These results are consistent with the hypothesis that these compounds are toxic in part due to their ability to undergo redox cycling and thereby generate reactive oxygen species. However, other modes of action, such as direct enzyme inhibition, may also be important.     

Histological and quantitative effects of sublethal cyanide exposure on oocyte development in rainbow trout

Sexually maturing female rainbow trout were exposed in the laboratory to 0.01 mg/L and 0.02 mg/L hydrogen cyanide (HCN) for 20 days at 10 ±0.5C. Histological sections of ovaries from these fish indicated abnormal cytoplasmic features among oocytes during their early development. Oocytes exposed to cyanide during late development exhibited altered patterns of secondary yolk deposition. Quantitative observations similarly confirm delays in secondary yolk deposition. Exposure of females to 0.02 mg/L HCN during early summer ovarian growth resulted in delayed secondary yolk deposition among 93% of the oocytes when compared with controls. When fish were exposed during mid-summer, the effects were evident at both concentrations with approximately 60% of the oocytes failing to reach the secondary yolk deposition stage at the completion of the experiment. This was further substantiated by potential growth studies which indicated a complete absence of oocytes in the final stages of secondary yolk deposition at both cyanide concentrations as compared with controls which contained 27%. The high frequency of atresia at both concentrations suggested that oocytes unable to produce secondary yolk were resorbed by the ovary through a unique form of atresia. It is also suggested that levels of cyanide as low as 0.01 mg/L have a critical effect upon the process of secondary yolk deposition within the ovary and thus significantly reduce the number of viable eggs.     

Sublethal and acute toxicity of the ethylene glycol butyl ether ester formulation of triclopyr to juvenile coho salmon (Oncorhynchus kisutch)

The toxicity of Garlon^®4, the ethylene glycol butyl ether ester formulation of the herbicide tryclopyr, to juvenile coho salmon (Oncorhynchus kisutch) was investigated at several lethal and sublethal concentrations. Fish behavior, random activity and oxygen uptake were monitored. Coho salmon exhibited three distinct responses related to concentration and duration of exposure: (1) at concentrations greater than 0.56 mg/L fish were initially lethargic, then regressed to a highly distressed condition characterized by elevated oxygen uptake and finally death, (2) at 0.32–0.43 mg/L fish were lethargic throughout the exposure period with reduced oxygen uptake, and (3) at concentrations 0.10 mg/L fish were hypersensitive to stimuli, exhibiting elevated activity and oxygen uptake levels during photoperiod transitions. Whole body residue analysis showed that uptake of the ester and subsequent hydrolysis to the acid form in the fish was rapid, with significant accumulation of the acid in the tissues. This suggests that some threshold tissue concentrations were associated with the observed results. For juvenile coho salmon the 96-hr LC₅₀ of Garlon^®4 was 0.84 mg/L.     

Effect of ethyl acetate on carbohydrate metabolism of common Indian catfish

Median lethal concentration (LC₅₀), LC₀, and LC₁₀₀ values of ethyl acetate for common Indian catfish, Heteropneustes fossilis, were estimated at 24, 48, 72, and 96 hr with 95% confidence limits. The 96-hr LC₅₀ value and harmless (safe) concentration were 212.5 and 63.89 ppm, respectively. Exposure of the fish to 170 ppm of ethyl acetate for 3, 6, 12, 48, and 96 hr induced marked changes in carbohydrate metabolism. Hepatic glycogen values declined significantly at 3, 48, and 96 hr, but there was no marked alteration in muscle glycogen content at any of the exposure periods. Hyperglycemia occurred at all time intervals. Blood pyruvate levels were elevated at 3, 6, 48, and 96 hr. Hyperlacticemia resulted at 3 and 96 hr but hypolacticemia occurred at 6 and 12 hr. Impairment of carbohydrate metabolism might be responsible for the toxic action of ethyl acetate.     

Toxicity of selected organic pesticides to a fresh water teleost fish,Saccobranchus fossilis and its application in controlling water pollution

Bioassay studies have been conducted with chlordane (an organochlorine compound), and Ekatin?, Ekalaux?, and Sumithion? (organophosphorous compounds) to determine the LC₅₀, acute toxicities, relative toxicities, presumable harmless concentrations, and dilution volumes for a period of 24-, 48-, 72-, and 96-hr to a fresh water teleostSaccobranchus fossilis. The 24-, 48-, 72-, and 96-hr LC₅₀ for chlordane, Ekalaux, Ekatin, and Sumithion are 0.54, 0.49, 0.46, and 0.42 mg/L; 2.75, 2.40, 1.75, and 1.55 mg/L; 12.40, 12.10, 11.75, and 11.00 mg/L; and 14.90, 14.20, 13.75, and 12.50 mg/L, respectively. Presumable harmless concentrations are 0.12, 0.55, 3.46, and 3.87 mg/L, respectively, which are a fraction of LC₅₀ values. The relative toxicities of these pesticides calculated on the basis of 24-hr LC₅₀ to the fish are 27.59, 5.42, 1.20, and 1.00 for chlordane, Ekalaux, Ekatin, and Sumithion, respectively. The acute toxicity for Sumithion to the test fish lies between 12.10 and 18.75 mg/L, for chlordane between 0.37 and 0.75 mg/L, for Ekatin between 10.00 and 14.20 mg/L, and for Ekalaux between 1.35 and 3.25 mg/L. It is concluded from the study that chlordane is the most toxic and Sumithion is the least toxic while the toxicities of Ekatin and Ekalaux lie between the two. When the data for survival of the test fishvs. log concentration are plotted, they exhibit a linear relationship following the formula Y= ˉY+b(X−ˉX). The presumable harmless concentrations can be utilized on broad lines for controlling short-term water pollution by these pesticides.     

Comparisons of Acute Toxicity of Selected Chemicals to Rainbow Trout and Rats

Median lethal concentration (LC₅₀) and dose (LD₅₀) for 217 chemicals were compared over several exposure routes in rainbow trout and rats. Data were compiled primarily from on-line databases and included octanol–water partition coefficient (K_ow), trout LC₅₀, and trout and rat LD₅₀for oral, dermal, and intraperitoneal (IP) routes.K_owand molar lethality data were log-transformed for correlation and regression analyses. Median ratios (rat oral LD₅₀/trout LC₅₀=156, rat/trout oral LD₅₀= 0.80, rat/trout dermal LD₅₀=0.43, rat/trout IP LD₅₀=0.55) demonstrated the effect of similar versus dissimilar exposure routes on lethality, reflecting toxicokinetic differences. Simple linear regressions revealed significant correlations (BonferroniP<0.05) for trout LC₅₀versus rat oral LD₅₀and for trout LD₅₀versus rat LD₅₀when matched on exposure route. WhenK_owwas included with multiple regression to consider the effect of hydrophilic/lipophilic partitioning on uptake and toxicity, correlations improved only for trout LC₅₀versus rat oral LD₅₀and for trout IP LD₅₀versus rat IP LD₅₀. Stratification of data into pesticides and nonpesticides failed to refine prediction of trout LC₅₀from rat oral LD₅₀. Simple regressions reported here and in the literature relating fish LC₅₀and rat oral LD₅₀exhibited varying degrees of correlation (r=0.26–0.99), reflecting differences in interspecies sensitivity, chemical group evaluated, and experimental methodologies.     

Acute toxicity of hydrogen cyanide to freshwater fishes.

Acute toxicity of hydrogen cyanide was determined at various temperatures from 4 degrees to 30 degrees C and oxygen concentrations of 3.36 to 9.26 mg/L on different life history stages of five species of fish: fathead minnow, Pimephales promelas Refinesque; bluegill, Lepomis macrochirus Rafinesque; yellow perch, Perca flavescens (Mitchill); brook trout, Salvelinus fontinalis (Mitchill); and rainbow trout, Salmo gairdneri Richardson. Median lethal threshold concentrations and 96-hr LC50's were established by flow-through type biassays. Acute toxicity varied from 57 microgram/L for juvenile rainbow trout to 191 microgram/L for field stocks of juvenile fathead minnows. Juvenile fish were more sensitive at lower temperatures and at oxygen levels below 5 mg/L. For most species juveniles were most sensitive and eggs more resistant.     

Partition distribution of insecticides as a critical factor affecting their rates of absorption from water and relative toxicities to fish

Loaches,Misgurnus anguillicaudatus (Cantor), a common fish in Taiwan, were treated with DDT, dieldrin, and monocrotophos by continuous exposure in aqueous solutions (or suspensions) and by injection. DDT and dieldrin were 150 and 220 times more toxic, respectively, than monocrotophos, to the fish exposed in aqueous solutions (24-hr LC₅₀), but only 1/9 and 1/4 as toxic as monocrotophos by injection (24-hr LD₅₀). Results of GLC analyses indicate that, at the end of 24-hr exposure, 96.5% of DDT, 92.7% of dieldrin, and 14.3% of monocrotophos were absorbed by loaches from aqueous solutions. The initial rates of absorption for DDT and dieldrin were about 10 to 20 times faster than that for monocrotophos. The large differences in relative toxicity may be due to partition distribution which in turn caused differences in absorption, as DDT and dieldrin are lipophilic and monocrotophos is hydrophilic. Statistical analysis of the relationship between fish toxicities and partition coefficients supports the present finding. The coefficient of correlation is 0.70 between partition coefficients (benzene/water) and toxicities to fish (rainbow trout) of 12 organophosphorus insecticides, 0.74 between coefficients and corrected fish toxicities, and 0.96 between partition coefficients and corrected fish toxicities for organophosphates only. Results of analyses are significant at <1% probability level. Similar correlation was also obtained between partition coefficients for hexane/water and toxicities of 8 organophosphorus and 5 organochlorine insecticides to rainbow trout.This investigation was supported by grants from the Joint Commission on Rural Reconstruction, Republic of China.For reprints, please write to the Plant Protection Center, Taiwan, Taichung Hsien, Taiwan 431, Republic of China.This paper constitutes partial fulfillment of requirements for the M.S. degree of the senior author at the National Taiwan University.     

Toxicity of anin situ oil shale process water to rainbow trout and fathead minnows

Anin situ oil shale process water, designated Omega-9 water, was used in flow-through bioassays with fathead minnows, rainbow trout and rainbow trout eggs. Of the two fish species, rainbow trout were more sensitive to acute exposure to Omega-9 water with 96-hour LC₅₀ dilutions of 0.51% and 0.41% in two independent determinations. In embryo-larval studies, the length of fry from eggs hatched and maintained in 0.16% process water was significantly less than that of eggs hatched in control water. A solution of the 13 major inorganic constituents of Omega-9 water, with a 96-hour LC₅₀ of 0.56% for rainbow trout, showed that inorganics accounted for most of the acute toxicity of Omega-9 water.Work funded under an Interagency Agreement between the U.S. Department of Energy and the U.S. Environmental Protection Agency under Contract No. DE-AS20-79 LC 01761 to the Rocky Mountain Institute of Energy and Environment, University of Wyoming.     

Environmental impact of mosquito pesticides: Toxicity and anticholinesterase activity of chlorpyrifos to fish in a salt marsh habitat

Chlorpyrifos was tested for its influence on thein vitro andin vivo brain acetylcholinesterase (AChE) activity ofFundulus heteroclitus under laboratory as well as field conditions. The concentration required for a 50% reduction in the mvitro enzyme activity (I₅₀) was 7.2 × 10^–2 mM for the parent compound and 4.1 × 10^–6 mM for its oxygen analog. A 96-hr exposure of live fish to 1.0g/L chlorpyrifos resulted in a maximum AChE inhibition of 24%. At a concentration of 2.1g/L or higher, a 100% enzyme inhibition was observed after a 24-hr exposure period, followed by varying degrees of recovery during the next 24 hr. A second peak of AChE inhibition, proportional to the concentration of the insecticide, was observed 72 hr after the initial exposure and this was followed by a second phase of recovery during the next 24 hr. In spite of an initial 100% AChE inhibition at test concentrations of 2.1g/L and above, the fish mortality was less at lower concentration of the insecticide than at the higher levels. The 96-hr TL₅₀ and TL₅ (tolerance limit for 50% and 5% fish survival) of chlorpyrifos forF. heteroclitus were 4.7g/L and 12.2g/L, respectively. The LT₅₀ (lethal time in which 50% of the fish died) at a concentration of 5.6g/L was 49.5 hr.Fish exposed to 4 successive field applications of chlorpyrifos granules showed AChE inhibition ranging from 56 to 100% and the insecticide effect was cumulative in nature. By 24 hr after the second application, 18.6% of the treatment fish had died; live fish collected at this time showed a 96% depression of AChE activity. AChE inhibition was still evident (62%) in fish 69 days after the final application of chlorpyrifos.Paper of the Journal Series, New Jersey Agricultural Experiment Station, Rutgers-The State University of New Jersey, New Brunswick, New Jersey 08903, USA. Supported by a grant from N. J. State Mosquito Commission.     

Comparative toxicity of offshore and oil-added drilling muds to larvae of the grass shrimpPalaemonetes intermedius

Offshore drilling fluids (muds) varied widely in their toxicity to grass shrimp (Palaemonetes intermedius) larvae. The 96-hr LC₅₀s for the eleven drilling muds tested ranged from 142 to >100,000 ppm (l/L). There was a significant correlation between oil content of the drilling muds and their toxicity. Furthermore, addition of diesel oil (No. 2 fuel oil) or mineral oil to an offshore drilling mud having a low oil content or to an oil-free synthetic drilling mud led to a marked increase in the toxicity of these muds. Thus, much of the toxicity of the offshore drilling muds tested can be attributable to the oil content.