The mechanism inducing milk secretion in 10 puerperal women and in their children (Witch's milk) was studied by means of sequential hormone measurements starting at delivery and continued during the first postpartum week. Determinations included prolactin (PRL), growth hormone (GH), estradiol 17β(E217β), and progesterone (PG). Hormonal levels in the newborns closely paralleled those of the mothers. In both groups, the onset of milk secretion coincided with the disappearance of sex steroids from plasma in the presence of elevated PRL concentrations. Response to TRH (8 μg/kg) revealed adequate PRL reserve, and failure of this peptide to modify basal GH in the neonates was suggestive of appropriate hypothalamic control. The role of estrogens in the induction of pituitary hyperplasia during pregnancy is discussed.