Prolactin levels were measured by radioimmunoassay in umbilical cord plasma from fetuses, in capillary plasma from neonates, and in venous plasma from adults. The concentrations of prolactin in cord plasma from fetuses having gestational ages of 16 to 19 weeks, 20 to 34 weeks, and 35 to 42 weeks were 53 +/- 16 (mean and SE), 233 +/- 30 and 371 +/- 7 ng/ml, respectively. The prolactin levels decreased to 218 +/- 35 ng/ml during the first neonatal week. The similarity of the patterns of prolactin levels, reported gestational estrogen levels and adrenal weights, as well as the known biological properties of prolactin, estrogen, and ACTH is consistent with the view that these three factors may be involved in the growth of the fetal adrenal cortex and its involution in the newborn.