Gonadal function was evaluated in 10 men [33 +/- 17 (SD) yr] with pituitary tumors and hyperprolactinemia (47-2550 ng/ml) using nocturnal penile tumescence (NPT), semen analysis, urinary LH and FSH excretion, and diurnal variation of serum testosterone and PRL. Results were compared to 16 normal subjects (33 +/- 13 yr). NPT was decreased in tumor patients as demonstrated by reduced maximum circumference change (P less than 0.01) and total tumescence time (P less than 0.05). Semen analysis was examined in 5 patients able to produce specimens. All seminal parameters were significantly abnormal as demonstrated by oligospermia, asthenospermia, teratospermia , and elevated fructose. Urinary LH [570 +/- 72 (SE) vs. 838 +/- 22 mIU/h; P less than 0.01] and serum testosterone (235 +/- 60 vs. 625 +/- 63 ng/dl; P less than 0.01) were decreased in 9 tumor patients, all of whom had serum PRL levels above 50 ng/ml. Diurnal variation of serum PRL was absent in hyperprolactinemic patients whereas all had normal circadian changes in serum testosterone, although at a lower set point. Eight patients were followed for 6-13 months after reduction of serum PRL by surgery and/or drug therapy. Serum PRL reached normal levels in six men after 6 months of treatment. Selected individuals had an increase in serum LH after 2 months of treatment. Significant rises in serum testosterone occurred as early as 3 months and normal levels were found in six patients after 6-8 months of treatment. Only two subjects, however, demonstrated a normal semen analysis. These data suggest that men with serum PRL levels above 50 ng/ml maintain a normal diurnal pattern of serum testosterone at a lower set point, and demonstrate hypogonadism with reduced urinary LH excretion and NPT. In addition, routine seminal parameters are clearly abnormal and are both delayed and incomplete in their recovery.