In the present studies, the content and the in vitro production of prolactin by decidua as well as the concentrations of prolactin in amniotic fluid, maternal and fetal serum in normal term pregnancies, induced abortions at various gestational ages, and in pregnancies complicated by diabetes mellitus, preeclampsia, chronic hypertension, and polyhydramnios were measured. Maternal and fetal prolactin levels varied considerably throughout gestation, but at term did not differ significantly between normal and abnormal pregnancies. Prolactin levels in amniotic fluid as well as decidual prolactin content and production were significantly lower only in pregnancies complicated by either hypertension or polyhydramnios. In both normal and abnormal pregnancies, decidual prolactin production correlated strongly with amniotic fluid concentrations. The present data suggest that 1) maternal and fetal prolactin levels do not differ significantly between normal and abnormal pregnancies, 2) the decidua is the principal source of amniotic fluid prolactin, and 3) the significantly lower levels of prolactin in amniotic fluid of pregnancies complicated by hypertension or polyhydramnios are probably due to adverse effects of these conditions on the synthesis and release of prolactin by decidua.