A specific and sensitive radioimmunoassay method for oxytocin and its application to the measurement of human plasma oxytocin levels are described. Antiserum tooxytocin was prepared in guinea pigs by repeated subcutaneous injections of synthetic oxytocin emulsified in complete Freund's adjuvant. The labeled oxytocin was purified by a complex procedure with gel filtration on Sephadex G-10 column and salt gradient column chromatography on DEAE-Sephadex A-25. Separation of antibody-bound hormone from free hormone was performed by a double antibody method. Sensitivity of the assay was 0.02mIU/ml and arginine-and lysine-vasopressins showed no competition in this assay system. The recovery of the unlabeled oxytocin added to the late pregnancy plasma was 61.5±7.0 per cent, without any extraction procedure.Plasma oxytocin levels during human labor are also described. An evident correlation was found in plasma oxytocin levels between maternal and cord venous blood (r=0.74). Oxytocin levels in cord plasma were significantly higher than in maternal plasma. During the second stage of labor, no significant changes of oxytocin levels were noted in maternal peripheral venous blood.