Heat tolerance was assessed by magnitude of strain induced in the body as a whole by heat load. The strain was represented by a combination of relative water loss, relative rise in rectal temperature and relative salt loss, using those critical values of the three factors which cause heat stroke (40.6 degrees C), water depletion heat exhaustion (7% of body weight) and salt depletion heat exhaustion (0.75 g per kg of body weight). As this numerical heat tolerance index is defined as the degree of disturbance induced by heat exposure concerning thermal regulation, water and electrolyte metabolism, the magnitude of the index is inversely proportional to that of heat tolerance. Our studies showed that adaptive changes in heat tolerance of unacclimatized subjects during short-term heat acclimatization could be followed up by this heat tolerance index and that superior heat tolerance of residents in subtropical zone and athletes could by reasonably evaluated by the index. Thus, it might be said that this numerical heat tolerance index is a reliable index for the assessment of heat tolerance.