Young sedentary adult males of Malay, Indian, and Chinese origin who had established continuous residence in tropical Malaysia and presumed to be naturally acclimatized to heat, were studied to evaluate their physiological responses to a standard heat stress test. The Malay and Indian races have evolved in hot and humid geographical zones, whereas the Chinese originated from a temperate area. Subjects exercised at 50% VO2max alternating 18 minutes walking and 2 min rest during a 2-h exposure to an ambient of 34.9 degrees C dry bulb and 32.1 degrees C wet bulb. Heart rates, core and skin temperatures, sweat rates, and oxygen uptakes were measured during the heat exposure. The subjects of Malay origin exhibited the least circulatory stress of the three ethnic groups. The data obtained on these long-term residents of a hot-wet climate and who were considered acclimatized to this environment were compared to experimental data obtained by other investigators and other ethnic groups.