A long-acting form of bromocriptine, a prolactin (PRL) secretion inhibitor, was administered to 122 postpartum women in single intramuscular injections of 20 (N = 24), 30 (N = 22), 40 (N = 46), and 50 mg (N = 30). In 91 women the substance was administered immediately after delivery to prevent galactopoiesis and in the remaining 31 women to inhibit established lactation. Effectiveness was estimated by the absence of breast engorgement and of milk secretion. Successful prevention or inhibition of lactation was highest among women receiving 50 mg bromocriptine (97%), and comparison between dosages revealed a close linear dose-response relationship (r = 0.98). Persistent and significant (P less than .001) PRL inhibition could be recorded for up to 22 days in successfully treated puerperas in comparison with 12 normally breast-feeding women who served as control subjects. No significant side effects or local reactions were recorded. Eleven of 46 postpartum women receiving 20 or 30 mg bromocriptine experienced onset of milk secretion or lactation rebound, and responded to oral administration of the drug. The presence of milk was associated with plasma PRL concentrations persistently above 25 ng/mL in all of them, whereas nine women in the same dosage range in whom lactation suppression was effective exhibited values below this limit. Dose-response data allow the establishment of a putative PRL threshold for induction of milk secretion of about 25 ng/mL. Maintenance of plasma PRL values below this limit prevents lactogenesis and inhibits lactopoiesis.