The topography of eccrine sweating in humans during exercise

The purpose of this study was to investigate the distribution of steady-state sweating rates (m _sw), during stressful exercise and heat exposures. Six men completed 42-min trials: 2-min rest and 40-min cycling at 40% peak power in 36.6° C (relative humidity 46.0%). The m _sw, was monitored using ventilated capsules at the forehead, and at three additional sites. Repeat trials allowed monitoring from eleven skin surfaces. Auditory canal temperature (T_ac) and 11 skin temperatures were measured. After normalising m _sw to the forehead response within subjects, differences in T _ac and onset time thresholds, and transient and steady-state m _sw were examined. The pooled, lower torso m _sw onset [mean 45.5 (SEM 42.0) s] preceded that of the head [mean 126.5 (SEM 34.8) s, P<0.05], but was not significantly different from the legs [mean 66.6 (SEM 25.7) s], upper torso [mean 80.2 (SEM 36.8) s] or arms [mean 108.6 (SEM 31.2) s]. Transient m _sw did not differ among regions (P=0.16). Mean, steady-state forehead m _sw [3.20 (SEM 0.51) mg · cm^–2 · min^–1]was not significantly greater than the scapula, forearm, hand, stomach and lower back m _sw (in descending order), but was greater than the chest [1.6 (SEM 0.2)], upperarm [1.6 (SEM 0.2)], calf [1.5 (SEM 0.3)] and thigh m _sw [1.0 (SEM 0.2), P<0.05 for all comparisons]. The results did not support the caudal-to-rostral sweat onset evident during supine, resting heat stress. Equivalent T _ac sweat thresholds existed between sites, while steady-state m _sw topography varied among subjects and was not dominated by central regions.     

The role of spinal thermosensitive structures in the respiratory heat loss during exercise

Summary Two dogs were prepared with spinal thermodes and hypothalamic guide tubes. The spinal thermodes could be used either for measuring the temperature of the peridural space of the spinal cord or for artificially altering this temperature. The hypothalamic guide tube was for measuring hypothalamic temperature. It was found that during exercise the temperature of the spinal cord increased and closely followed the temperature of the working muscles. The temperature of the hypothalamus increased only insignificantly. In a second series of experiments the spinal cord temperature of the same two dogs was artificially altered during exercise and the change in respiratory evaporative heat loss (REHL) was measured. It was found that spinal cord warming increased REHL both at rest and during exercise. At air temperatures of 30° and 32°C the sensitivity to spinal cord warming, as judged from the intercept and slope of calculated regression lines, was unaffected by exercise. At air temperatures of 25° and 27°C the sensitivityto spinal cord warming during exercise was the same as at the higher air temperature.—Spinal cord cooling was unable to inhibit the REHL during exercise. It is therefore concluded from these results that temperature signals generated in the spinal cord are not involved in the control of exercise induced increase in REHL.     

Determination of body heat storage in clothing: calorimetry versus thermometry

Two methods of estimating body heat storage were compared under differing conditions of clothing, training, and acclimation to heat. Six male subjects underwent 8 weeks of physical training [60–80% of maximal aerobic power ( ) for 30–45 min · day–, 3–4 days · week^–1 at < 25 °C dry bulb (db)] followed by 6 consecutive days of heat acclimation (45–55% for 60 min · day^–1 at 40°C db, 30% relative humidity)]. Nine other male subjects underwent corresponding periods of control observation followed by heat acclimation. Before and after each treatment, subjects walked continuously on a treadmill (1.34 m · s^–1, 2% grade) in a climatic chamber (40°C db, 30% relative humidity) for an average of 118 min (range 92–120 min) when wearing normal light combat clothing and for an average of 50 min (range 32–68 min) when wearing protective clothing resistant to nuclear, biological, and chemical agents. The heat storage was determined calorimetrically (by the balance of heat gains and losses) and thermometrically [by the conventional equations, using one or two set(s) of relative weightings for the rectal temperature (T _re) to mean skin temperature _sk of 4:1 and 4:1, 2:1 and 4:1, or 2:1 and 9:1 in thermoneutral and hot environments, respectively]. _sk was calculated from 12-site measurements, weighted according to the regional distribution of body surface area and the first eigenvectors of principal component analysis. There were only minor differences (< 5%) between the heat storage values calculated by given weighting factors forT _re and _sk, whether the individual coefficients were derived from estimates of regional surface area or principal component methodologies. When wearing normal clothing, no significant differences were found between the two estimates of heat storage (calorimetry vs thermometry with an invariant relative weighting of 4:1) in any experimental condition, with one specific exception: when wearing protective clothing, thermometry underestimated the heat storage by 24–31%. This underestimation was attenuated by using two sets of relative weightings of 2: 1 and 4: 1 or 2: 1 and 9: 1. The results suggest that when subjects wearing protective clothing are transferred from thermoneutral to hot environments, the accuracy of thermometric estimates of heat storage can be improved by using two sets of weighting factors forT _re and _sk     

Effect of a low-carbohydrate diet on plasma and sweat ammonia concentrations during prolonged nonexhausting exercise

The purpose of this investigation was to examine the effect of low body glycogen stores on plasma ammonia concentration and sweat ammonia excretion during prolonged, nonexhausting exercise of moderate intensity. On two occasions seven healthy untrained men pedalled on a cycle ergometer for 60 min at 50% of their predetermined maximal O₂ uptakes ( _max) firstly, following 3 days on a normal mixed diet (N-diet) (60% carbohydrates, 25% fat and 15% protein) and secondly, following 3 days on a low-carbohydrate diet (LC-diet) (less than 5% carbohydrates, 50% fat and 45% protein) of equal energy content. Blood was collected from the antecubital vein immediately before, at 30th and at 60th min of exercise. Sweat was collected from the hypogastric region using gauze pads. It was shown that plasma ammonia concentrations after the LC-diet were higher than after the N-diet at both the 30th and 60th min of exercise. Sweat ammonia concentration and total ammonia loss through the sweat were also higher after the LC-diet. The higher ammonia concentrations in plasma and sweat after the LC-diet would seem to indicate an increased ammonia production, which may be related to reduced initial carbohydrate stores.     

Predictors of sweat loss in man during prolonged exercise

Summary Nineteen healthy male subjects, differing in training status and (52±1 ml · min^–1 · kg^–1, mean ±SEM; 43–64 ml · min^–1 · kg^–1, range), exercised for 1 h at an absolute workload of 192±8 W (140–265 W); this was equivalent to 70±1% (66–74%). Each exercise test was performed on an electrically braked cycle ergometer at a constant ambient temperature (22.5±0.0° C) and relative humidity (85±0%). Nude body weight was recorded prior to and after each exercise test. Absolute sweat loss (body weight loss corrected for respiratory weight loss) during each test was 910±82 g (426–1665 g); this was equivalent to 1.3±0.1% (0.7–2.2%) of pre-exercise body weight (relative sweat loss). Weighted mean skin temperature and rectal temperature increased after 5 min of exercise from 30.5±0.3° C and 37.2±0.1° C respectively to 32.5±0.2° C and 38.8±0.1° C respectively, recorded immediately prior to the end of exercise. Bivariate linear regression and Pearson's correlation demonstrated absolute sweat loss was related to (r=0.72,p<0.001), absolute exercise workload (r=0.66,p<0.01), body surface area (r=0.62,p<0.01), weight (r=0.60,p<0.01) and height (r=0.53,p<0.05). Relative sweat loss was related to (r=0.77,p<0.001) and absolute exercise workload (r=0.59,p<0.01). There was no relationship between sweat loss (absolute or relative) and heart rate, skin temperature or rectal temperature. In addition, there was no relationship between rectal temperature or absolute exercise workload or . Stepwise multiple linear regression indicated to be the most important predictor of absolute (r=0.72,F=18.27,p<0.001) and relative (r=0.77,F=24.58,p<0.001) sweat loss in man during prolonged exercise.     

Effects of dietary sodium on body and muscle potassium content during heat acclimation

Summary It has been suggested that renal conversion of sodium (Na⁺) during training in hot environments results in potassium (K⁺) deficiencies. This investigation examined the influence of two levels of dietary Na⁺ intake (399 vs 98 mmol · d⁻¹) on intramuscular, urinary, sweat, and whole body K⁺ homeostasis. Nine unacclimated, untrained males underwent heat acclimation during two 8 day dietary-exercise regimens (40.1±0.1‡ C, 23.5±0.4%RH). Both diets resulted in depressed urinary K⁺ excretion. Sweat K⁺ and muscle K⁺ concentrations were not altered by diets or acclimation. The whole body stores of Na⁺ increased 31.1% (+ 916.8 mmol) during the high Na⁺ diet and decreased 7.8% (−230.4 mmol) during the low Na⁺ diet; whole body stores of K⁺ increased 4.1% (+ 137.6 mmol) during the high Na⁺ diet and increased 3.4% (+ 113.6 mmol) during the low Na⁺ diet. This dietary-acclimation protocol did not result in whole-body or intramuscular K⁺ deficits and offers no evidence to support previous claims that dietary sodium levels affect K⁺ balance.     

Acid-base balance and subjective feelings of fatigue during physical exercise

Six trained male subjects performed exercise on a bicycle ergometer. The external load was increased every minute by 10 watt until exhaustion. The subjects quantified their subjective feeling of fatigue by means of a rating scale.Parameters of acid-base balance (pH, ) were determined in arterial blood from the a. brachialis.Correction of the acidaemia by infusion of NaHCO₃ (8%) during exercise had no effect upon the subjective feeling of fatigue, and except for carbon dioxide output no effect upon some important physiological functions (heart rate, blood pressure, ventilation, and oxygen consumption) during submaximal and maximal exercise.     

Responses of young and older men during prolonged exercise in dry and humid heat

Summary Eight young, sedentary men (aged 34 years, SD 3) and six older moderately active, unacclimated men (aged 57 years, SD 2) walked on a treadmill at 30% of their maximum oxygen consumption up to 3.5 h in a thermoneutral [dry bulb temperature (T _db) 21°C, relative humidity (r.h.) 43%)], a warm humid (T _db 30°C, r.h. 80%) and a hot dry (T _db 40°C, r.h. 20%) environment while wearing ordinary working clothes (0.7 c/o). Their oxgen consumption, heart rate (f _c), rectal (T _re) and mean skin temperature (T_sk), sweat rate (SR), and evaporative rate (ER) were measured during the tests. The ratings of thermal sensation (TS) and perceived exertion (RPE) were assessed using standard scales. In the heat stress tests, the number of experiments discontinued did not significantly differ between the two groups. The mean levels and end-exercise values of T _re, T_sk, f _c, TS and RPE were not significantly different between the young and older subjects in any of the environments. In the warm humid environment, however, the T _re and RPE of the older subjects increased continuously (P<0.05) during the test compared to the young subjects. No significant difference between the groups was observed in SR or in ER. In the hot dry environment, however, the ER of older men increased more slowly compared to the young men. In spite of some time-related differences observed in T _re, RPE, and ER, the older subjects did not exhibit higher f _c during exercise in the heat, they were not more hyperthermic and their performance times were similar to the young subjects. Therefore, it was concluded that older calendar age is not necessarily associated with a reduced ability to exercise in a hot environment and other factors, such as physical activity habits and aerobic capacity, may be equally important in determining heat tolerance in the elderly.     

Effects of facial cooling during exercise at high temperature

Summary Healthy men and women were exercised on a cycle ergometer in a hot environment (46 C). Cold air (5 C) was blown onto the face either as a jet from the tube directed towards the nose or being introduced under a face mask. There was a subjective feeling of increased comfort although the temperature under the mask decreased to only 28 C–32 C. Facial cooling did not cause any changes in either blood pressure or heart rate. It is concluded that this degree of facial cooling does not invoke the previously described peripheral vasconstriction which could inhibit further heat loss.     

The reproducibility of closed-pouch sweat collection and thermoregulatory responses to exercise–heat stress

Seven active male subjects cycled for 60 min at 29.5 (0.8)% peak work rate on three separate occasions in a hot environmental condition [36.0 (0.1)°C, 60 (1)% relative humidity] in order to determine the reproducibility of a closed-pouch sweat collection technique for sweat composition at the scapula, forearm and thigh. To confirm that sweat composition was not influenced by between-trial variations in sudomotor drive, local sweat rate, whole-body sweat rate, heart rate (HR), rectal temperature (T_re) and mean skin temperature (T_sk) responses were also measured, consequently reproducibility was also established for these variables. Sweat composition did not differ among trials, with the mean coefficients of variation (CVs) for sweat [Na⁺], [K⁺] and pH being 10.4 (7.4)%, 8.1 (6.5)% and 1.3 (1.1)%, respectively. Local sweat rates did not differ among the three trials (P>0.05) although whole-body sweat rate was reduced in the third trial (P<0.05). The mean CVs were 11.0 (7.8)% and 4.7 (1.6)% for local and whole-body sweat rates, respectively. Between-trial differences were not evident for T_re, T_sk or HR with mean CVs of 0.3 (0.2)%, 0.7 (0.6)% and 3.9 (1.7)%, respectively, although HR tended to be greater in the first trial (P=0.08). It is proposed that moderate variations in sweat composition were influenced by variations in the local sweat rate, which were induced by application of the pouch.     

Carbohydrate and fat metabolism of unacclimatized men during and after submaximal exercise in cool and hot environments

Summary Changes in levels of plasma lactate, pyruvate, glucose, free fatty acids (FFA), glycerol and 3-hydroxybutyrate during muscular exercise and recovery in cool (ambient temperature T _a = 23 C and water vapor pressure P _w = 13 mb) and hot environments (T _a = 40 C, P _w = 30 mb) were measured in six subjects. Arterial blood samples were collected at fixed time intervals during an initial resting phase of 75 min, an exercise period of 20 min and a recovery period of 125 min. Exercise consisted in pedaling a bicycle ergometer in the supine position at a work load that raised the heart pulse rate to nearly 140 beats min^–1 in the cool condition. Heart rate and rectal temperature were significantly higher during exercise and recovery in the hot environment. For samples collected at corresponding times in the cool and the hot conditions, the deviations of the observed blood parameters from their mean values during the whole resting phase seldom differed. However, for the exercise phase as a whole, the mean increases of lactate, pyruvate and glycerol were significantly greater and the mean decrease of glucose significantly smaller, in the hot condition, while the mean decreases of FFA and 3-hydroxybutyrate did not differ significantly. Related to their corresponding mean resting values, the mean changes in all six blood parameters measured were significantly larger during recovery in the hot condition than in the cool. These results verify the enhanced anaerobic metabolism in hot environment already described by previous authors, and show an adipokinetic and hyperglycemic effect of acute exposure to heat.     

Thermoregulatory stress during rest and exercise in heat in patients with a spinal cord injury

Summary Twelve subjects with spinal cord injuries and four controls (all male) were exposed to heat while sitting at rest or working at each of three environmental temperatures, 30, 35 and 40°C, with a relative humidity of 50%. Exercise was accomplished at a load of 50 W on a friction-braked cycle ergometer which was armcranked or pedalled. Functional electrical stimulation of the legs was provided to the subjects with quadriplegia and paraplegia to allow them to pedal a cycle ergometer. The data showed that individuals with quadriplegia had the poorest tolerance for heat. As an example, in this group, accomplishing armcrank ergometry while working at an environmental temperature of 40°C resulted in an increase in aural temperature of 2°C in 30 min. The aural temperature of individuals with paraplegia working for the same length of time under the same conditions rose approximately 1°C. There was virtually no change in the aural temperature in the control subjects.     

A suit calorimeter for energy balance studies on humans during heavy exercise

A modification of the suit calorimeter originally developed in 1972 was used in combination with indirect respiratory calorimetry. The modification included increased cooling capacity of the suit by means of an increased density of cooling tubes and a variable water flow pump which permitted higher flow rates. This has made the suit calorimeter a very effective heat exchanger that could be used for studies on high energy turnover during heavy exercise. Furthermore, specially designed absorption clothing made it possible to collect any sweat produced before it evaporated, thus minimizing potential error in measuring evaporative heat loss. The suit calorimeter would seem to offer a valuable tool in the analysis of the specific thermogenic responses to dietary changes and physical activity in studies on energy and protein metabolism and their interaction in humans. It also makes it possible to perform direct calorimetric measurements in metabolic balance studies using continuous parenteral infusion since the subjects do not need to be sealed in a calorimeter chamber.     

Sodium replacement and fluid shifts during prolonged exercise in humans

In the study presented here, we examined the affects of a close to complete replacement of sweat water and Na⁺ losses on fluid shifts during exercise. Six cyclists performed three 4-h rides at 55% of their peak oxygen uptake in a 20°C environment while consuming 3.85 l of an 8% carbohydrate solution containing 5, 50 or 100 mEq·l^–1 of Na⁺. Increases in Na⁺ intake reduced renal free water clearance from around 40 ml·h^–1 to –8 and –121 ml·h^–1 and led to a decrease in urine volume from ≅1.0 to 0.5 l (P<0.05). In contrast, the 3.5–3.9 l fluid and 150–190 mEq Na⁺ losses in sweat were similar in each trial, as were the ≅80 mEq K⁺ losses in sweat and urine and the 282–288 mosmol·kg^–1 plasma osmolalities. During the low-Na⁺ trial, plasma osmolality was maintained by a ≅1.3 l contraction of extracellular fluid (ECF) with the loss of ≅200 mEq Na⁺. However, in the other trials, ≅1.3 l of water was lost from the intracellular fluid. During the medium-Na⁺ trial, a loss of only ≅40 mEq Na⁺ maintained ECF volume, and during the high-Na⁺ trial, a gain of ≅160 mEq Na⁺ expanded the ECF by ≅0.8 l. However, corresponding changes in plasma volumes from –0.20 to 0.15 l had no effect on cardiovascular drift or thermoregulation. These data suggest that during prolonged exercise of moderate intensity under mild environmental conditions when sweat rates are ≅0.9 l·h^–1, complete Na⁺ replacement maintains plasma volume and reduces dehydration, but when fluid intake matches sweat rate, has little effect on plasma osmolality. Electronic Publication     

The effects of different levels of heat production induced by diathermy and eccentric work on thermoregulation during exercise at a given skin temperature

Summary The thermal responses of two healthy male subjects have been studied at the same mean skin temperature (T _sk ) during negative work, positive work and positive work in which additional heating was induced by diathermy. The results showed that for a given metabolic heat production (M) rectal (T _re ) and oesophageal (T _oes ) temperatures were higher in negative work and positive work with diathermy than normal control experiments. In resting experiments with diathermy, T _oes rose to the same level as when an equal amount of heat was produced metabolically by exercise. In negative work and positive work with diathermy sweat loss (M _sw ) was higher for a given M and T _sk than found for normal exercise, but in all three forms of work the relationship of M _sw to total heat production (H) was identical. During positive work with and without diathermy the differences in M _sw could be accounted for by using a previously developed model of relative sweating rate: %M _sw = – constant + T _re (or T _oes ) + T _sk .In negative work, removal of the difference between predicted and observed %M _sw required the inclusion of a further factor into the equation based on muscle temperature. The results suggest that the core temperature in exercise rises to meet the requirements of heat dissipation mainly by stimulating M _sw and establishing a heat transfer gradient from core to periphery and is not necessarily or uniquely related to M or to the rate of working. The study underlines the usefulness of negative work and diathermy as physiological tools for the further understanding of thermoregulation during exercise.     

Hormonal responses to opioid receptor blockade: during rest and exercise in cold and hot environments

Opioid receptors appear to modulate a variety of physiological and metabolic homeostatic responses to stressors such as exercise and thermally extreme environments. To more accurately determine the role of the naloxone (NAL) sensitive opioid receptor system during rest and exercise, subjects were subjected to concomitant environmental thermal stress. Fifteen untrained men rested or performed low intensity (60% ) or high intensity (80% ) exercise on a cycle ergometer for 60 min in an environmental chamber during cold (0°C) hot (35°C) air exposure while receiving an infusion of normal saline (SAL) or NAL (0.1 mg kg⁻¹). Plasma adrenocorticotropin hormone (ACTH), immunoreactive beta-endorphin (IBE), cortisol and growth hormone were measured at baseline and every 15 min while in the chamber. Time to exhaustion was significantly reduced during high intensity exercise in the heat (P<0.0001). NAL significantly (P=0.0004) reduced the time to exhaustion (38.3±2.1 min) during high intensity exercise in the heat compared to SAL (49.4±2.1 min). ACTH and IBE increased during hot conditions and cold attenuated this response. Plasma concentrations of IBE, ACTH, and growth hormone increased significantly with NAL during high intensity exercise in the heat compared to SAL. Cold attenuated the response of ACTH, IBE and cortisol to NAL. NAL administration exaggerates plasma hormone concentration during high intensity exercise in the heat, but not cold. These results support a regulatory effect of the opioid receptor system on physiological responses during exercise in thermally stressful environments. Future research should be directed to more clearly defining the effect of environmental temperature on the mechanism of hypothalamic–pituitary–adrenal hormonal release during exercise and hot environmental temperatures.     

A modified,local sweat collector for warm and humid conditions

Summary The purpose of this study was to modify a previously described local sweat collector to facilitate the investigation of sweat rate and composition in a warm (30°C) and humid (relative humidity 80%) environment. The adherence of the collector to the skin was improved and a pouch was appended at the lower end of the collector. The limitations of the closed collector were examined by comparing the local sweat rate and the quantity of electrolyte excreted in sweat with those obtained using a second collector with a wide opening (to permit free evaporation) and by changes in the body mass. Eight subjects performed exercise on a cycle ergometer consisting of four equal periods of 15 min each, at 60% maximal oxygen consumption, with a rest of 5 min between each period. The sweat produced on a local skin area (85 cm², upper posterior thorax region) was collected at the end of each period, before measuring the body mass on a sensitive (±1 g) platform balance. The mean local sweat rate [2.61 (SEM 0.19) Mg-CM^–2. min^-1] was 2.4 times greater than the pro-rated whole body mass loss but the two were strongly correlated (r=0.82,P<0.01). Compared to the open collector, the greater quantity of electrolyte excreted into the closed collector would suggest that the conditions which prevailed in the closed collector, such as a higher local skin temperature, may have affected the function of the sweat gland. This method enabled the efficiency of local sweat evaporation to be assessed by measuring the difference between sweat volume collected in the open and in the closed collectors. Recovery of water volumes at rest indicated that no contamination and no apparent leakage occurred. This improved sweat collector is suitable for obtaining clean local sweat samples of up to 6 ml, and for measuring the sweat composition and also sweat rate during exercise in warm and humid conditions.     

2,3-diphosphoglycerate during exercise

Summary The effect of physical exercise on erythrocyte 2,3-diphosphoglycerate (2,3-DPG) content was studied in eight trained speed canoeists aged 15–25 years. One-stage 4-min load on a paddling ergometer corresponding to a 1 km race was observed. During the test, the following cardiorespiratory parameters were recorded: . Blood lactate level and haematocrit were also determined. The average resting values of 2,3-DPG and haematocrit were statistically significant increased.After 6 month intense training there was a significant increase in 2,3-DPG, and the other parameters were also increased.Abbreviations 2,3-DPG 2,3-diphosphoglycerate - Hc haematocrite - pulmonary ventilation - oxygen uptake - oxygen uptake per one kg of body weight - HR heart rate - pulse oxygen - R respiratory quotient - La lactate - ¯x arithmetical mean - SD standard deviation - t value of t-test for paired comparisons     

Relationship between ventilation and arterial potassium concentration during incremental exercise and recovery

Summary The purpose of this study was to compare the relationship of ventilation ( _E) with pH, arterial concentrations of potassium ([K⁺]_a), bicarbonate ([HCO₃ ^–]_a), lactate ([la]_a), and acid-base parameters which would affect hyperpnoea during exercise and recovery. To assess this relationship, ten healthy male subjects exercised with intensity increasing as a ramp function of 20 W · min^–1 until voluntary exhaustion and they were then allowed a 5-min recovery period. Breath-by-breath gas exchange data, [HCO₃ ^–]_a, pH, [1a]_a, [K⁺]_a and blood gases were determined during both exercise and recovery. Using a linear regression method, the _E/[K⁺]a relationship was analysed during both exercise and recovery. Several interesting results were obtained: a significant relationship between [K⁺]_a and _E was observed during recovery as well as during exercise; the _E at any given values of [K⁺]_a was significantly higher during recovery than during exercise and out of those factors affecting exercise hyperpnoea, only [K⁺]_a had a similar time-course to _E during recovery. Changes in [K⁺]a during recovery were shown to occur significantly faster than _E with an [K⁺]_a time constant of 70.0 s, SD 16.2 as opposed to 105.5 s, SD 10.0 for _E (P < 0.01). These results provided further evidence that [K⁺]_a might play an important role as a substance which can stimulate exercise hyperpnoea as has been suggested by other workers. The present study also showed that during recovery [K⁺]_a contributed significantly to the control of _E.     

The effects of immersion and exercise on prolactin during pregnancy

Summary Prolactin is an important hormone during pregnancy, affecting mother, fetus, and amniotic fluid volume. Immersion is known to affect prolactin levels significantly. To determine the effect of immersion and exercise on the prolactin response during pregnancy, we examined serum prolactin levels at 15, 25, and 35 weeks' gestation and 10 weeks post partum. Twelve women completed 20 min land rest, 20 min immersion in 30° C water to the xiphoid, and 20 min exercise in the water at 60% Resting prolactin levels were 1.91±0.32, 4.55±0.5, and 5.85±0.27 nmol·l^–1 ±standard error of the mean at 15, 25, and 35 weeks' gestation, respectively. Postpartum lactating women had a resting mean prolactin level of 3.95±1.6 versus 0.22±0.4 nmol· l^–1 in non-lacting women. Prolactin levels declined significantly during immersion even after correction for dilution by plasma volume shifts. The immersion response was inversely related to the duration of pregnancy with 29%, 22%, and 12% drops during 15-, 25- and 35-week trials, respectively. Compared to rest, exercise prolactin levels remained depressed during the 15th and 25th week trials. We hypothesize that immersion in water caused prolactin levels to decline.