Effects of dehydration (3% of initial body weight) on temperature regulation were investigated in 5 men during intermittent exercise of 4 h duration at a dry air temperature of 34 degrees C. Relative mechanical work load was 50% of the subject's steady state heart rate, which was 170 beats . min-1. During rehydration from the 70th min to the end of the exercise, the subjects drank, every 10 min in equal portions, an amount of water (20 degrees C) totaling up to 80% of the body weight loss recorded during dehydration runs. Continuous measurements were made of rectal (Tre) and mean skin (Tsk) temperatures and of whole body weight loss. Chest sweating rate (msw) was measured from a capsule located under a local thermal clamp (36 degrees C). Blood samples were obtained during rest periods and after the 1st and the 4th hour of exercise. Compared to dehydration runs, water intake did not always cause an increase of msw while body temperatures always decreased. Dehydration resulted in a decrease in plasma volume and in increases of plasma osmolality, [Na+] and [K+]. Water intake induced a thermoregulatory response whose intensity largely differs from one body area to another. The change in the slope of the relation of msw to Tre features a decrease in the sensitivity of the thermoregulatory system with dehydration. The whole body water loss is significantly correlated with the change in plasma volume and body temperatures (Tre, Tsk). This suggests that the reduced sweating response observed during dehydration can be related to plasma hypovolemia.