Freshly obtained human placentas from various periods of gestation were quantitatively analysed for their immunoreactive oxytocin (OT) content and its biological activity was examined in a Magnus apparatus by utilizing rat uterus. The mean values for placental immunoreactive OT per gram tissue increased from the first to the second trimester, maintaining its high level to term. The total content of placental OT also increased continually from the beginning of pregnancy to term. Blood levels of estrogen stimulated neurophysin (ESN) and OT were concomitantly enhanced through gestation. Placental extract and synthetic OT showed similar peaks in the elution pattern of ion-exchange chromatography through a carboxymethyl cellulose column. Synthetic OT and placental extract induced marked uterine contraction in diestrous rats. However placental extract previously incubated with OT antiserum failed to induce this effect. Though detection of immunoreactive OT by immunoassay alone does not provide definite identification of pituitary and placental OT, the present study suggests that placental immunoreactive OT could have a contracting effect on the uterine muscle.