1. This paper describes a computer model (SCENARIO) specifically designed to simulate the time course of heat strain observed during athletic, industrial, and military settings.2. The simulations generated by the model dependably reproduce the time course of body temperature shifts, thermoeffector responses, central and peripheral circulatory changes in persons exercising in warm and hot environments.3. The model takes into consideration different clothing ensembles, varied levels of aerobic fitness in a population, and effects of progressive dehydration and deconditioning (extended bed rest).