In the group of 18 normoprolactinemic patients stimulated by human menopausal gonadotropins (HMG), serum prolactin (PRL) and progesteron (P) levels were evaluated at the time of estradiol (E2) increase before, during and after chorionic gonadotropin (HCG) administration. In 8 patients (group I) mean serum PRL levels remained normal, ranging from the mean basal values (12.4 +/- 1.7 micrograms/l) to the mean values at the time of HCG administration (16.8 +/- 1.8 micrograms/l) and after it (24.9 +/- 6.2 micrograms/l). In 10 patients (group II) PRL concentration reached pathologic values (from 12.0 +/- 1.8 micrograms/l in basal conditions to 38.9 +/- 5.2 micrograms/l at the time of HCG administration and after it 39.9 +/- 5.4 micrograms/l). The mean serum P concentration at the time of E2 peak was higher in the second group (2.8 +/- 1.0 nmol/l) than in the first group (1.47 +/- 0.2 nmol/l) but the difference was not significant. Four days after HCG administration a significant increase of P levels was registered in both groups of patients (184.1 +/- 8.9 nmol/l in the first group and 205.6 +/- 34.8 nmol/l in the second group). Considering these analyses we can presuppose that transitional hyperprolactinaemia does not impair early steroid production in the corpus luteum of HMG stimulated cycles.