RESUMEN
Physiological responses to heat and heat tolerance were examined in summer and winter on 13 male athletic university students and male nonathletec university students. After staying for 30 min. in a climatic chamber maintained at 30°C with 70% relative humidity, sweating reaction was examined far 90 min, by immersing both legs up to the knees in a stirring water bath of 42°C.<BR>Both groups showed significantly greater sweat volume, significantly lower Na concentration in sweat and considerably lower rise in rectal temperature and less increase in heart rate in summer than in winter. In both seasons, athletes showed smaller volume of sweat, lower Na concentration in sweat, lower rise in rectal temperature and less increase in heart rate than nonathletes.<BR>It is concluded that heat tolerance of athletes was superior to that of nonathletes when assessed by our heat tolerance indices and this superior heat tolerance of athletes could be explained due to a result of physical training. Heat tolerance index, representing the magnitude of physiological strain in the body induced by heat load, was modified by using relative increase in heart rate in place of salt loss. It can be said that the modified heat tolerance index is useful as a substitute of the original heat tolerance index in field studies.