Research indicates that endurance exercise training has significant effects upon the reproductive endocrine system of humans. Until recently, this effect was thought to be limited primarily to women. However, a growing body of evidence demonstrates that the male reproductive endocrine system is also effected. Specifically, the circulating hormonal levels of testosterone are found to be at low concentrations; and, the hypothalamic-pituitary-testicular axis that regulates testosterone production is altered in endurance trained men. The physiological mechanism inducing the lower testosterone is currently unclear; but in many respects, these men display hypogonadotropic hypogonadism characteristics. Currently, the time course of the changes in the reproductive endocrine system is unresolved and in need of much furthers scientific investigation. The evidence available, however, suggests that a slowly developing process requiring years of exercise training results in these changes. Potentially, the lowered testosterone levels of the endurance-trained male could disrupt some of their anabolic or androgenic dependent processes. To date, there are only a limited number of findings suggesting that a consistent disruption of testosterone dependent processes occur due to endurance exercise training (e.g., oligo-spermatogenesis). Conversely, the alterations in testosterone concentration brought about by endurance training could have cardiovascular protective effects and thus be beneficial to the health of these men.