Investigations on reproductive function in male athletes are not as abundant in the literature as the research available on female athletes. The primary reason for this is the absence of an obvious clinical sign indicative of an alteration in reproductive function in male athletes. While alterations in the reproductive status of female athletes may be easily detected by the loss of menstrual regularity, a distinctive clinical sign reflective of reproductive dysfunction in the male is not apparent. In male runners, an effect of endurance training on reproductive function related to a specific 'volume threshold' of training is proposed. Data are supportive of this 'volume threshold' effect, provided careful and consistent definitions of volume of training are applied. In fact, if volume of training is carefully defined endurance-trained male runners exhibit a rather consistent range of subclinical modifications in the gonadal hormones and semen profile, and clinical (oligospermia) alterations in reproductive function. The precise mechanism responsible for these observed alterations remains unknown, although several peripheral and central mechanisms have been suggested. Clearly, more research is necessary to confirm, and to elucidate, the nature of the 'volume threshold' hypothesis in male runners.