Blood cortisol, ACTH, thyroxine, triiodothyronine, reverse triiodothyronine, thyroid stimulating hormone (TSH) [thyrotropin] and vasopressin concentrations were determined in 9 runners (29-56 yr old) and one 80 yr old man taking part in a non-competitive marathon. After the run the mean concentrations of cortisol, ACTH and vasopressin showed a significant rise. The thyroid function variables and TSH did not differ from the control values. There was a significant correlation between the cortisol and ACTH levels after the race and also between their increments from the corresponding base values. A significant correlation was found between the physical fitness (as measured by indirect determination of .ovrhdot.VO2 max and the post-race cortisol levels. One of the well-trained runners with a fairly good running time had the highest post-race values for 6 of 7 hormones studied.