Hyperpyrexia or heatstroke has been recognized for many years as a risk of workers exposed to high environmental heat loads, particularly when they fail to use available protective measures. Although heatstroke has been studied in detail along epidemiological1as well as physiological lines,2it has not been considered much of a problem in ill patients during a period of sustained heat where there are no provisions for air conditioning. When troops move suddenly into hot, dry deserts; warm, moist, jungles; or tropical conditions, particularly after the confined inactivity of a sea voyage, heat exhaustion is the usual form of heat disability. Heat exhaustion commonly is nothing more than unrecognized dehydration. Intake of water promptly corrects it. Hyperpyrexia from heat occurs when the sweat glands are worn out or overwhelmed acutely. Both problems have been studied in the natural environment as well as under experimental conditions of the laboratory.