In two separate experiments 8 healthy young men were given an egg-milk formula at a level of 0.7 g protein/kg per day, and were exposed to high environmental temperature (34--37 C, 7 hours daily) or cool temperature (21--26 C, all day), alternately. In the first experiment a 16-day hot period was followed by a 20-day cool period and finally by a 20-day hot period. Daily urinary nitrogen (N) loss of the last 8 days of cool period, 82.2 mg/kg, was significantly higher than that of the last 8 days of the 20-day hot period 69.7 mg/kg. Daily skin N loss was significantly lower during the last 8 days of the cool period (3.5 mg/kg) than during the last 8 days of the 20-day hot period (11.9 mg/kg). Urinary N and skin N lossess were negatively correlated (r equal to -0.905) in these periods. In the second experiment a 28 day hot period was followed by a 20-day cool period. Skin N loss diminished from 12.3 mg/kg daily during the last 12 days of the 28-day hot period to 4.1 mg/kg during the last 12-days of the cool period. At the same time, urinary N loss increased significantly from 81.4 mg/kg during the 28-day hot period to 95.2 mg/kg during a cool period. Urinary N and skin N losses were again negatively correlated (r equal to -0.620) during these periods. Results of these studies indicate that when skin N loss increases during high temperature, urinary N loss decreases gradually, but total N loss does not increase.