Six normally menstruating women who regularly run participated in a 10-mile race. Blood samples were collected within 20 minutes after the completion of the race (group 1). Samples were analyzed, and the results were compared with plasma hormonal concentrations in the same runners in samples collected between 12 and 24 hours after a previous practice run (group 2) and with our nonathletic female controls. Plasma concentrations of the following hormones in group 1 were significantly elevated when compared with group 2: dehydroepiandrosterone (DHA), androstenedione (delta 4A), testosterone (T), cortisol (F), luteinizing hormone (LH), and prolactin (PRL). Levels of 17 beta-estradiol (E2), 17-hydroxyprogesterone (17-OHP), dehydroepiandrosterone sulfate (DHAS), and follicle-stimulating hormone (FSH) were comparable in groups 1 and 2. However, DHAS in group 1 was elevated when compared with controls, as were HDA, delta 4A,T,F,LH, and PRL. In group 1, but not in group 2, a significant correlation (P less than 0.05) was observed between plasma LH and PRL concentrations but not between FSH an PRL. We conclude that the immediate effect of running is reflected in increased levels of the adrenal androgens, F, LH, and PRL. However, concentrations of these hormones revert back to baseline within 12 to 24 hours after the race.