PIP: Recent studies have confirmed the presence of a separate (HPr) human prolactin molecule. Measurement of HPr concentrations has been performed in normal and abnormal gestation and during the menstrual cycle. HPr rises throughout gestation with a return to prepregnant concentrations by the 7th postpartum day. Variable increases in plasma prolactin were observed after intravenous administration of arginine, especially after the 30th week of gestation. Puerperally, prolactin rises in response to sucking. Amniotic fluid prolactin levels are 100 times those of maternal or fetal blood. No significant change in basal HPr is seen during the menstrual cycle. The ability to measure this new pituitary polypeptide by radioimmunoassay permits investigation of its role in human gestation.