A prospective study was made on the duration of lactation in 406 mothers injected postpartum for contraception with medroxyprogesterone acetate (DMPA), 150 or 250–300 mg, IM, at 3-or 6− month intervals. Breast feedings up to 2 per day were recorded, and findings compared with those from 173 nontreated controls. Eighty percent of the DMPA group were nursing successfully at the sixth month and 42% at the 12th month postpartum. These figures are significant and compare favorably with the control group. No advantage was found in the administration of DMPA earlier than the second or third month after delivery. Amenorrhea persisted during lactation. No pathologic breast conditions were observed. The eventual mechanism of DMPA in galactopoiesis has yet to be clarified.