The hourly rates of sweating of men walking on a treadmill in severe heat declined steadily during the course of 6-hr. expts. The men were well acclimatized to the heat and maintained water balance by drinking 0.1% saline in the experiments. In 50 expts. the avg. rate of sweating during the first 2 hrs., i.e., initial rate, was 1400 g./hr. and the sweating rates of the men declined from 10-80% of this value by the 6th hr., depending upon environmental conditions. The decline occurred only in relatively high rates of sweating since the men were able to sweat at a practically constant rate (about 750 g./hr.) while working in moderate heat. The declines of the sweating rates were distinctly greater in humid than in dry heat where the initial rates of sweating were about equal. Also, in both humid and dry heat the decline was greater when the men wore Army tropical uniforms than when they wore only broadcloth shorts. Another response shown by men wearing tropical uniforms was a greater decline in their rates of sweating as the initial sweating rates increased. Since the decline in sweating was not associated with dehydration nor with a decreased strength of the stimulus for sweating, it is concluded that the sweating mechanism was fatigued in some way.