This study examined the effects of acute and chronic pyridostigmine bromide (PB) administration on thermoregulatory and metabolic responses to exercise in cold air (5 degrees C). Seven healthy men completed two 7-day trials in a double-blind, crossover experimental design: during one trial they received PB (30 mg three times daily) and during the other trial they received placebo. For each trial, subjects attempted four (3 h) exercise tests: low-intensity exercise (approximately 25% VO2max) and moderate-intensity exercise (approximately 50% VO2max), on days 2 and 3 and again on days 6 and 7. Metabolic rate, body temperatures, and venous blood samples were obtained before and during exercise. Red blood cell acetylcholinesterase inhibition induced by PB increased (p < 0.05) from 34% on day 1 to 43% on days 3-7. Metabolic rate, body temperatures, and regional heat conductance responses were not different between trials. Plasma glucose, glycerol, free fatty acid, lactate, sodium, and potassium concentrations were not different between trials. In addition, differences were not found between acute and chronic experiments for any thermoregulatory or metabolic responses. These findings demonstrate that the PB dosage used by military personnel, as a pharmacological defense against nerve-agent poisoning, should not cause any adverse thermoregulatory or metabolic effects during moderate activity in cold climates.