Five subjects exercised at an ambient temperature of 6.2°C, five exercised at 37.7°C, and five served as non-exercise controls. Each subject in the exercise groups performed one 45-min treadmill run a day for five consecutive days at heart rates that represented 80% of their maximum capacity. Sperm production was quantified the week preceding treatment and from 6–10 wk post-treatment. Plasma testosterone levels (PTL) were measured before each exercise test and immediately, 30 min, and 60 min after the first and fifth tests. The initial pre-exercise sample was taken immediately preceding the first test. Rectal temperature (Tre) and heart rate were monitored during all tests. No significant differences were found for sperm production. Plasma testosterone levels were 32.9% greater on the fourth and fifth day of testing than they were on day one (P< 0.018). The 6.2°C group (1202.7 ng%) had higher resting PTL than either the 37.7°C (897.3 ng%) or the control (968.8 ng%) groups throughout the exercise period (P< 0.001); PTL were 46.6% greater on day 5 than they were on day 1 for the 6.2°C group (P< 0.021). The change in Tre attributed to exercise was significantly greater in the 37.7°C group (2.5°C) when compared to the 6.2°C group (1.1°C;P0.001). In summary, the changes in PTL and Tre that occurred as a result of 5 d of heavy exercise did not affect sperm production rates in humans.