The effects of administration of norethindrone 1 mg with mestranol 0.08 mg to 50 nursing mothers are described. The effect on lactation was inhibitory when compared to controls, demonstrated by lower infant weight gains in spite of an increase in supplemental calories given to the infants. The duration of lactation was decreased significantly when compared to 50 controls. This effect was more pronounced in primiparas when compared to those with previous lactation success. The medication had no obvious effect on involution of infant breast tissue. No deleterious effect on maternal genital involution was noted.