Prolactin (PRL) secretion in the periparturitional period in patients undergoing labor and vaginal delivery follows a remarkable multiphasic pattern not found in patients who underwent elective cesarean section without labor. There is a highly significant decline in PRL levels during active labor which reaches a nadir about two hours prior to delivery. Immediately after delivery, a surge of PRL is noted, reaching peak levels within two hours post partum. Thereafter, PRL levels fall, reaching a second nadir about nine hours post partum, and this low level is maintained for nine to 24 hours after delivery. This multiphasic pattern of PRL secretion is not correlated with changes in serum concentrations of cortisol, progesterone, estradiol, or estrone. PRL levels in all pregnant patients at term were unaffected by the administration of synthetic narcotic analgesic agents, anesthesia, or the stress of operation. It is concluded that PRL secretion in the pregnant patient at term is unresponsive to usual stimuli and that the multiphasic pattern of PRL secretion uniquely found with labor and vaginal delivery may be associated with dopaminergic neuroendocrine processes during human parturition.