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To assess the lethal effects of work-induced hyperthermia on exercising animals, untrained rats were run to exhaustion at 5 and 20-26 degrees C or restrained at 41.5 degrees C. An exercise-induced core temperature of 40.4 degrees C represented a base line above which mortalities occurred. With increasing core temperature at exhaustion (between 40.4 and 43 degrees C), mortality increased within 24 h. A dose-respones curve with an LD50 equivalent to a core temperature of 41.5 +/- 0.1 degrees C was calculated. Although differences in body weight loss, core temperature at exhaustion, and cooling rate will clearly distinguish between survivors and fatalities, the severity of heat injury as inferred from survival times is best measured by the time versus intensity of hyperthermia in degree-minutes.

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