The response of total testosterone (T), free testosterone (fT), sex hormone-binding globulin (SHBG) and non-SHBG-bound testosterone (NST) to the same exercise protocol was evaluated in two different experiments on long distance runners. The first experiment was performed in the morning at 0900 hours with nine athletes, while the second was carried out in the afternoon at 1500 hours with seven athletes. During each experiment, each athlete ran for 1 h at the previously determined speed corresponding to 2 mmol.l-1 blood lactate concentration. Three venous blood samples were collected in each experiment: before exercise, at the end of running and after 1 h of recovery. Total T and SHBG showed similar responses: in the first experiment they had decreased after exercise, while in the second they had increased at the end of running. A positive correlation between total T and SHBG concentrations was found at the end of exercise. In both experiments, NST and fT had increased after exercise and decreased to initial concentrations during recovery. The results would suggest the existence of a compensatory mechanism which maintains adequate concentrations of biologically active T when total T concentrations decrease.