Serum prolactin concentration was measured by radioim- munoassay in 29 women, In the first 1½ hours postpartum. Fourteen women received 0.2 mg methylergonovine maleate (Methergine) intramuscularly after the delivery of the placenta. Fifteen women who served as controls received only saline. The rise in serum prolactin concentration seen in the control women (266.4 ng/ml ± 40.8 SE) was significantly greater than that seen in methylergonovine-treated patients (141.0 ng/ml ± 29.0 SE).