Eight Japanese men and women participated in this study. They were randomly exposed to two environments: hot-dry; HD (Ta=40°C, rh 30%, wet bulb globe temperature (WBGT)=32°C) and hot-wet; HW (Ta=31°C, rh=80%, WBGT=32°C) for 110 min. During the exposure, they rested on a bicycle ergometer for 20 min during rest and 30 min during recovery, then they pedaled it with an intensity of 40% Vo₂ max for 60 min. Tre, Tsk, and HR were recorded every minute. Total sweat loss and dripping were measured by independent bed balances which was connected to a computer processing with an accuracy of 1 g throughout the experiment. Sweat sodium concentration at forearm and back sites were collected by sweat capsule technique. These results showed that ΔTre, Tsk, evaporated sweat, dripping sweat, body heat storage of both sexes in HD were significantly higher than these in HW during exercise. HR of men in HD at the end of recovery was slightly higher than that of women. Whereas the sweat sodium concentration at forearm and back sites in both sexes remained unchanged either in HD or HW environment, it was found that HD was more stressful than HW environment under equivalent WBGT.