MURRAY, R. Nutrition for the marathon and other endurance sports: environmental stress and dehydration. Med. Sci. Sports Exerc., Vol. 24, No. 9 Supplement, pp. S319-S323, 1992. Key Points: 1) During exercise, the body's ability to safely regulate internal temperature is influenced by the environment, exercise intensity, clothing, and the athlete's level of fitness and acclimation. 2) Effective thermoregulation during exercise in the heat requires the evaporation of sweat. The onset of sweating during exercise is triggered by an increase in core body temperature. 3) Dehydration compromises cardiovascular and thermoregulatory function, limits physical work capacity, and increases the risk of heat-related health problems. 4) Physiological and behavioral adaptations help the body cope with the combined demands imposed by exercise and environment. 5) Ad libitum fluid intake is insufficient to protect against dehydration.