PIP: A study of the effects of IUD and hormonal contraceptives, both oral and injectable, on lactation is reported. The duration of lactation in 696 multiparous healthy women under contraceptive therapy was compared with the women's own previous duration of lactation when no contraception was used. In users of Lippes loop and injectable progestogens (in medroxyprogesterone acetate, DMPA) median duration of lactation (MDL) was significantly longer, even whem DMPA was injected immediately post partum, than in the controls. The MDL of Control Group B, which excluded those patients who had spontaneously stopped lactating within 30 days after delivery, was 5.3 months. The MDL in patients receiving DMPA 30 days after delivery was 9.3 months, and it was 6.7 months in those receiving DMPA immediately post partum. The MDL in patients with IUD was 7.7 months. When oral estrogen containing quinestrol and oral sequential steroidal contraceptives were given, MDL was significantly shorter than in control lactations. In the 2 groups of patients receiving the combination of quinestrol (Q1) and quingestronol acetate (Q2) orally, one a month, the MDL was 2.5 months respectively. The data indicate that compounds containing estrogens in the doses employed in this study should not be used as contraceptive therapy by nursing mothers.