Intramuscular injection of a single 50-mg dose of long-acting bromocriptine microspheres was compared with a single intramuscular dose of an estradiol/testosterone ester combination in a single-blind, randomized study of 54 subjects. Bromocriptine was significantly more effective than the steroid drug in preventing milk flow, and rebound lactation was not observed in any bromocriptine-treated patients. Neither group showed deleterious side effects or significant biologic changes in coagulation parameters. There were no blood pressure or electrocardiographic alterations. Postpartum prolactin suppression was more intense after bromocriptine administration than after steroid therapy.