In a randomized, double-blind study, 12 and 16 mg of metergoline and 5 mg of bromoergocriptine were given daily to three groups of 100 women each for ten consecutive days to prevent lactation. Excellent or good results were obtained in 86%, 90% and 83%, respectively. Serum prolactin (PRL) levels were evaluated in three settings. The first was after a single dose in four groups of patients (4, 6 and 8 mg of metergoline and 2.5 mg of bromoergocriptine were studied for ten hours). Second, for a study of the suckling reflex, PRL levels were measured 15, 30 and 60 minutes after breast stimulation in four groups of five patients each: two groups on the fourth postpartum day with and without metergoline treatment and two groups on the eighth postpartum day (the reflex was suppressed in the two groups treated with metergoline). Third, serum PRL levels rose after the intravenous administration of 200 mg of thyrotropin-releasing hormone (TRH) in another two groups of ten patients, one treated with metergoline and the other not so treated. TRH stimulation was not blocked in the treated groups.