1. To examine individual hormonal responses to extreme physical stress, blood samples were taken from eight highly trained athletes 1 day before and within 15 min of finishing the 1986 1000 km Sydney to Melbourne Ultramarathon foot race. 2. The baseline hormonal state of these highly trained athletes was quite different from normal. Resting serum conjugated catecholamines--epinephrine (E), norepinephrine (NE), dopamine (D), free E and free D--were significantly elevated above the normal mean (P less than 0.01). ACTH levels were significantly elevated above the normal range. Immunoreactive beta-endorphin (IR-beta EP), growth hormone (GH), prolactin (PRL), testosterone, cortisol and cortisol-binding globulin (CBG) were within the normal range. 3. The effect of the race on serum catecholamine levels was to elevate further free and conjugated NE (P less than 0.01). Other catecholamines, free and conjugated, remained significantly elevated above the normal mean (P less than 0.01). Adrenocorticotrophic hormone (ACTH) remained elevated, and IR-beta EP within the normal range, without significant change. A significant increase in GH (P less than 0.05), PRL (P less than 0.01), and cortisol (P less than 0.01) was seen, with no change in CBG. 4. As a model of chronic physical stress, the ultramarathon runner demonstrates a significantly altered baseline hormonal state as reflected in the primary mediators of the stress response, the catecholamines and the hypothalamic-pituitary-adrenal axis. Their response to severe exercise is distinct from that of untrained individuals in whom conjugated catecholamines decrease and ACTH increase. This may represent hormonal adaptation to prolonged stress.