Research indicates that endurance-trained men have lower basal testosterone concentrations than age-matched sedentary control men. The physiological cause for this finding is uncertain. Therefore, we examined the peripheral component in the hypothalamic-pituitary-testicular (H-P-T) axis in endurance-trained men to determine if their basal testicular production of testosterone was compromised. The study design was retrospective, with a case-control approach. Age-matched, trained (n=5, TRN) and sedentary control men (n=6, SED) were infused with gonadotrophin-releasing hormone (GnRH) to induce testicular testosterone production via subsequent luteinizing hormone elevations. Testosterone production rate was statistically analyzed with adjustments for confounding factors and compared between groups. The basal testosterone concentrations differed significantly between the TRN and SED groups [pooled mean values; 13.9 (3.0) nmol x l(-1) vs 23.4 (3.2) nmol x l(-1), P<0.05]. The testosterone production rate was significantly lower (approximately 20-30%; P<0.05) in the TRN men as compared with the SED men following GnRH infusion. It was concluded that the exogenous stimulated testicular production rate of endurance-trained men is suppressed. This finding may account, in part, for the lower circulating basal testosterone concentrations found in these men. The present evidence supports the hypothesis that endurance exercise training induces a degree of peripheral adaptation (i.e., testicle) in the H-P-T axis. Whether this adaptation in the axis is a permanent or transient phenomenon in these men remains to be determined.