The effect of Ablacton, Estrovis 4000, 2 Brom-alpha-ergokryptin (Cb 154) and of Lisurid-hydrogenmaleate (LHM) on lactation and the serum concentration of prolactin postpartum was studied. 10 normal nursing postpartum patients served as control. LHM was tested in a double blind study compared to placebo. The serum prolactin (PRL) was determined daily for the first 10 postpartum days by radioimmunoassay. Inhibition of lactation was consistent with Cb 154 (89% of the cases). Within two to three days after the onset of treatment with Cb 154 the changes of pregnancy in the breasts had completely subsided. The sexual steroids resulted in inhibition of lactation in 60% of the postpartum patients but mastodynia continued. LHM showed no difference from group taking placebo. The serum prolactin levels postpartum remained as high as a nursing mother's or rose with the administration of steroids. The administration of Cb 154 resulted in a drop of the serum prolactin to non-pregnant levels with in 2 days. The serum prolactin levels under treatment with LHM were not different from the group slowly. In nursing mothers, the stimulus of suckling maintain the pituitary secretion of prolactin as showed by higher serum prolactin levels and a slower decreased to normals than in the non-nursing mothers.