The hypothalamic-pituitary-testicular (H-P-T) axis was evaluated in groups of endurance-trained (TRG) and untrained (UNT) males. Each group was subjected to: 1) a 4-h resting hormonal profile [testosterone (T), free-testosterone (FT), estradiol (E2), luteinizing hormone (LH), prolactin (PRL), and cortisol (C)], 2) a dopamine antagonist (DA; 10 micrograms.kg-1 body weight) challenge to the pituitary-testes, and 3) a gonadotropin-releasing hormone (GnRH; 120 micrograms.kg-1 body weight) challenge to the pituitary-testes. Compared to UNT, the TRG resting T (4.8 +/- 0.7 vs 7.1 +/- 1.2 ng.ml-1, p = 0.05) and PRL (3.3 +/- 1.4 vs 7.0 +/- 2.3 ng.ml-1, p = 0.09) were lower while LH was elevated (15.0 +/- 1.8 vs 11.8 +/- 1.5 mIU.ml-1, p = 0.06). The DA challenge produced a greater integrated PRL response in the TRG (2962.7 +/- 265.1 ng +/- ml-1.min) than in the UNT (1735.3 +/- 282.0 ng.ml-1.min; p = 0.01). No significant changes were observed in T following the DA-induced PRL rise. The TRG had a blunted LH response (817.2 +/- 111.6 mIU.ml-1.min) following the GnRH injection as compared to the UNT (1493.7 +/- 213.4 mIU.ml-1.min; P less than 0.02). T levels were significantly (p less than 0.03) increased in both groups by the LH rise after the GnRH challenge (TRG = 9.9 +/- 5.0%; UNT = 8.6 +/- 9.9%, respectively), but no significant between group differences were observed. Results suggest endurance training produces an enhanced PRL and attenuated LH release by the pituitary.(ABSTRACT TRUNCATED AT 250 WORDS)