Physical activity, whether at a competitive or a recreational level, is frequently affected by environmental conditions. With changes in the natural environment, the exercising individual invokes a series of complex physiologic responses that are directed at maintaining homeostasis. This article will focus on the nature of these responses, with special reference to their influence on cardiorespiratory function. Because alterations in ambient temperature and ascent to altitude are the most common and are the most extensively studied of man's adaptational abilities, these will provide the basis for this discussion.