When a man is tranined in endurance exercise and subsequently heat exposed, the benefits of training are: (a) increased sensitivity of the sweat mechanism and (b) stability of circulating blood volume. The differences between a trained man before and after heat acclimatization are: (a) sweat mechanism that is not only more sensitive but provides adequate cooling for long periods and (b) an expansion of the vascular volume that is stable during heat exposure. Increased body temperature accompanies both training and acclimatization to heat; the main difference between the two states appears to be skin temperature. The importance of skin temperature in subsequent body fluid response to heat has not been investigated. Taking advantage of the fact that within a range of ambient temperatures from 50 degrees - 30 degrees C, core temperature during exercise (work) depends only on relative work load, I plan to study humans who have tranined at similar core temperature and with different skin temperature in order to assess the role of skin temperature in modification of bodily responses to heat stress.