In a group of 34 athletes and 11 untrained volunteers the plasma testosterone response to human chorionic gonadotropin (hCG) stimulation was evaluated. The hCG stimulation tests were performed by injecting 3000 IU of hCG i.m. on three consecutive days with plasma testosterone determinations before the first injection and 24 h after the last, i.e. third hCG injection. The basal testosterone levels were similar in both trained and untrained individuals. Plasma testosterone levels in response to repeated hCG stimulation were found to be significantly higher in the group of trained subjects.