The objective of the prospective randomized study reported here, based on 880 puerperae, was to study the effects of methylergometrin on involution, puerperal morbidity, and lactation. Over 4 weeks 444 mothers were given 0.125 mg of methylergometrin 3 times a day, while 436 were given the same dose of placebo. The following differences were found: in the treated group involution of the uterus was accelerated in the first few days following birth, but after 4 weeks there were no longer any significant differences. Post-partum pains were almost twice as intense in the treated group as in the untreated group. It proved impossible to reduce the number of cases of infection (lochiostasis, axillary temperature over 37.5 degrees C) by administering Methergin during the puerperium; following spontaneous births there were actually more cases of endometritis in the treated group. The number of patients with severe afterbleeding after spontaneous birth was also higher in this group. Lactation among untreated puerperae averaged 880 g during the first six days, while among treated patients it was only 563 g. Even after 4 weeks there were still differences in the quantity of milk produced. The incidence of infection and afterbleeding was significantly lower in mothers who breast-fed their children, irrespective of whether they had taken Methergin or not. We therefore consider that routine treatment of puerperae with methylergometrin is no longer justified; we continue to advocate breast-feeding, not least in view of the fact that it reduces maternal morbidity.