The PRL-inhibiting and ovulation-inducing effects of hydergine were studied in 18 patients with hyperprolactinemic ovulatory disturbances. The women were divided into 2 groups of 9 each. Those in group A had basal serum PRL levels higher than 100 micrograms/L, and those in group B had basal serum PRL levels lower than 100 micrograms/L. Serum PRL levels in both groups were determined before and hourly for 8 h after a single oral dose of either 2 mg hydergine or placebo. Hydergine induced a significant (P less than 0.01) decrease in the serum PRL level, compared with placebo, in both groups. The fall in mean serum PRL in group A was significant (P less than 0.05) 300 min after hydergine administration, and the concentration remained low at 480 min, at 45.5% of the mean baseline value. However, the serum PRL concentration did not reach the normal PRL range in any group A patient. Chronic treatment with this drug (2 mg, 3 times daily, for 4-12 weeks) in 5 patients from group A normalized the serum PRL concentration in only 1 patient and did not induce ovulation in any patient. In group B, also, hydergine administration significantly (P less than 0.01) reduced the mean serum PRL concentration within 240 min, and it declined further to within the normal range. The mean maximal reduction was to 56.4% of the baseline value at 360 min. All of the group B women received chronic treatment (2 mg, 3 times daily, for 4-104 weeks); repeated ovulation was induced in 7 (78%). Seven pregnancies ensued in 6 of the women in whom ovulation had been induced, and 4 normal infants were delivered. There were no side-effects, such as nausea and vomiting, during these trials. These data indicate that hydergine has PRL inhibitory activity and is useful in the treatment of hyperprolactinemic anovulatory patients whose basal serum PRL concentrations are below 100 micrograms/L.