Basal serum levels of prolactin (PRL) in 21 nursing mothers were measured by radioimmunoassay on the 2nd, 3rd and 4th days of the puerperium. The quantity of breast milk suckled during the 4th day of life was also evaluated by calculating the difference in the baby's weight before and after each feeding. During the first postpartum days, mean basal levels of PRL did not change. However these levels were noted to be significantly lower in the multiparas (p less than 0.05) than in the primiparas. In addition, the milk intake in neonates of multiparous mothers was significantly greater (p less than 0.05) than that in neonates of primiparous mothers. The author's hypothesis, based on the results of animal experimentation described in the literature, is that initiation of breast-feeding is facilitated in multiparas by the increased number of occupied PRL receptors in the mammary glands reflected by the lowered serum levels of the hormone.