A heat budget model to predict heat stress in humans during exercise is developed. The model considers the transfer of heat by conduction/convection from the skin (C), the latent heat due to evaporation from the skin (E), the sensible and latent heat exchanges due to respiration (CR and ER) and the heat gain due to solar radiation. Heat generated in the body core is conducted to the skin in accordance with a body conductance which is a function of the level of dehydration. The skin temperature is calculated assuming no heat storage, balancing C+E−R with the heat conducted from the body core to the skin. Heat that cannot be dissipated is assumed to elevate the core temperature. The model predicts the core and skin temperatures and moisture loss as a function of time. The model is used to define the maximum recommended duration of exercise (MRDE) as a function of ambient conditions and the level of exercise. Comparisons between daytime and nighttime conditions and light- and dark-colored clothing are made. MRDE charts for six different levels of exercise are given. It is suggested that these be used as guides in limiting exercise when the possibility of heat exhaustion is indicated.