Research has shown that chronic exposure to prolonged endurance training can result in disturbances within the reproductive physiological and endocrinological systems of humans. Until recently, research has focused on exercise training induced disruptions within female athletes. Within the last several years, however, studies have suggested that endurance training may have significant effects on the male reproductive system. The evidence suggests endurance training significantly affects the major male reproductive hormone, testosterone. At rest testosterone appears to be lower in the endurance-trained male than in the untrained male. The mechanism of this lowering is currently unclear, but may be related to dysfunctions within the hypothalamic-pituitary-testicular regulatory axis. This assumption has been based upon abnormalities in the resting testosterone levels and pituitary release of luteinising hormone and prolactin. Potentially, the lowered testosterone levels could disrupt reproductive and androgenic processes within the male. Presently, however, there are no findings to indicate that any consistent dysfunctions of any of the testosterone dependent processes in the male occur due to endurance training.