Circulating testosterone levels have generally been reported to increase during short term, intense running or bicycle ergometer exercise. Swimming differs from such activity because of the strenuous arm activity and because it is conducted in a liquid medium in the horizontal position with the weight totally supported. To investigate whether such differences modify the serum testosterone response to exercise, we measured serum testosterone levels before and after a maximal intensity swimming test in ten elite male and ten elite female swimmers. Levels of circulating testosterone fell in 19 of 20 swimmers in contrast to previous reports of exercise in the vertical position. In women mean testosterone levels declined by 39.4% from baseline values and in males, mean testosterone levels declined by 19.0%. Since the incremental protocol was similar to in design to maximal intensity tests conducted on a treadmill or bicycle ergometer, these data suggest that the differing physical circumstances of swimming lead to a qualitatively different testosterone reponse to exercise.