The change in plasma levels of testosterone and oestradiol-17-beta following a single intramuscular injection of 6000 IU hCG was studied in 7 men with non-tumoural hyperprolactinaemia and who were without clinical and/or laboratory evidence of hypogonadotrophism. These values were compared to the response of normal males to hCG. Plasma samples were obtained at 0, 2, 4, 24, 48, 96, 144, 192 and 240 h after hCG injection. The early and late phases of the plasma testosterone to hCG in hyperprolactinaemic patients were comparable to those of the controls, although the maximum and relative increment was somewhat diminished (5.0 +/- 1.2 vs 7.2 +/- 2.1 ng/ml; P less than 0.05, and 1.8 +/- 0.2 vs 2.3 +/- 0.5 ng/ml; P less than 0.02, respectively). The plasma oestradiol response to hCG was not different between the two groups, but the maximum and relative incement was higher in the hyperprolactinaemic patients (135.9 +/- 20.6 vs 97.1 +/- 11.9 pg/ml; P less than 0.05, and 4.9 +/- 0.6 vs 3.1 +/- 0.5 pg/ml; P less than 0.01, respectively. These findings suggest that the testosterone response to exogenous gonadotrophin is impaired in patients with non-tumoural hyperprolactinaemia. Whether this impairment is related to the higher increment in plasma levels of oestradiol remains to be elucidated. The data presented favour the hypothesis that in hyperprolactinaemic men, the prolactin exerts a direct action at the testicular level.