Exercise-training at environmental temperatures outside the thermal neutral zone, causes thermoregulatory adaptive reactions. In the heat, sweat efficiency is enhanced thus reducing the incidence of hyperthermia while in the cold discomfort and danger of frostbite are minimized by an improved cutaneous circulation specially in the extremities. Over long periods of time exercise does not affect thermogenesis except when refeeding. It is suggested that the reduced postprandial heat production reported in exercise-trained subjects is related to changes in nutrient metabolism which in turn could be related to insulin and norepinephrine modified actions.