The principal physiologic effect of PRL in human females is the initiation of lactation. The cellular events leading to actual secretion of breast milk are reasonably well defined in the experimental animal. Other than the direct effects on breast tissue in pregnancy, prolactin's role at puberty, during the menstrual cycle, and in pregnancy is poorly defined at present. Although there are large quantities of PRL in amniotic fluid the physiologic role is unknown. There is evidence for an osmoregulatory role of PRL in the amniotic fluid, and a very questionable role for PRL in fetal lung maturation. Overall, much more investigation is warranted to define more clearly the effects of PRL in reproductive physiology.