One hundred and ninetysix post-partum women, in whom lactation was to be prevented, were given under double-blind conditions either placebo or quinestrol 2 mg or 4 mg as a single oral dose within twentyfour hours of delivery. Early assessment of the results gave a failure rate of 58 per cent, 15 per cent and 5 per cent respectively, with statistically significant differences among the three groups of patients. At the follow-up evaluation, which could be made in only about one third of the women, breast troubles were recorded in 20 to 30 per cent, without significant differences among the groups. Post-partum amenorrhea showed a progressive prolongation from an average of 47.9 days in the controls to 64.6 and 72.6 days respectively in the 2 and 4 mg quinestrol groups. Adverse reactions, represented by delayed uterine involution during hospital stay and abnormal uterine bleeding in the late puerperium, were somewhat more frequent in the higher dose group than in the lower dose and control groups. On the basis of the prsent findings and of similar, though rare, experiences reported in the relevant literature, the question is therefore raised whether the 2 mg quinestrol dose would not be preferable to the 4 mg one for routine use in post-partum nonusing women.