Tritium-labelled testosterone was infused into four well-trained subjects at rest and during one hour of exercise at about 60% of their maximum aerobic power. This exercise regime led to a mean increase of 27% (range 10-51%) in plasma testosterone concentration. At the same time there were significant decreases in the estimated hepatic plasma flow (EHPF) (45%; range 28-67%), metabolic clearance rate of testosterone (MCRT) (29%; range 18-37%) and plasma volume (8.2%; range 3-10%). The production rate of testosterone decreased by 10% (range 9-22%) but this was not statistically significant. The ratio MCRT:EHPF increased in 3 out of 4 subjects in response to exercise but there was considerable inter-subject variation both at rest and during exercise. These findings suggest that the exercise-induced elevation of testosterone level is due solely to the reduction in the rate at which testosterone is cleared from the plasma. The principal cause of the reduction in MCRT is probably the reduction in EHPF but the variation in the ratio MCRT:EHPF suggests that changes in the extrahepatic clearance of testosterone may also be involved.