Water and salt requirements were studied in 17 and 18 healthy young men respectively. Each subject walked in a hot and humid environment (DB 28-34℃, RH 50-76%) at a speed of 5 km/hr with backpack loads of 15-25 kg for four hours. Dehydration would occur if no water or inadequate water was supplied and it would affect circulatory, sweating and thermore-gulatory function. It was suggested that MCHC= mightbeasensitive index to determine whether the body was dehydrated or not and to evaluate adequate amount of drinking water. The results showed that water loss should be replaced preferably by periodic intake of small amounts of water throughout the work period and the total amount required was about 80% of sweat loss. Thus, the rise of body temperature and heart rate was less than that of the subjects drinking 250 ml/hr of water or drinking voluntarily and there was little change in MCHC. The results also showed that the loss of salt by sweating was closely related to heat acclimatization. Therefore it is imporatnt to provide adequate salt to those unacclima-tized. Those taking 20 gm/day had a mild increase of rectal temperature and heart rate during work period and their urinary excretions of chloride were above 5 gm/day. According to the experiment, 20 gm/day of salt gave a satisfactory result.