Effects of sweat gland training by daily local heating were examined and its significance in heat acclimatization was evaluated. Training by 2-hr immersion of an arm in hot water of 43°C caused distinct augmentation of sweat gland activity in the trained area, with reduction in the degree of hidromeiosis, when tested by an arm bag collection of sweat. Concentrations of sweat electrolytes also showed definite decreases. The general tendency that Na and Cl concentrations in sweat rise in the course of hidromeiosis was attenuated or even reversed after the training. The sweat test using resistance hygrometry failed to show a marked or consistent increase in sweat rate of the trained area, although an increase was the common case on the dorsum of the hand and the extensor aspect of the forearm. The effect of training appeared in a few days of training, developed progressively with training days and showed a tendency to develop even after 3 weeks of training. The same training in midsummer failed to exert significant effects on sweat gland activity, suggesting that the sweat gland had been naturally trained to a considerable degree by then. On the other hand, training by repeated radiant heating of a local area caused only obscure changes in the activity of sweat glands. The present results reveal that sweat glands can be trained to be resistant to hidromeiosis in a hot-humid environment and that such peripheral changes appear to play a predominant role in augmentation of sweating capacity in the early stage of heat acclimatization.