Radioimmunoassayable plasma oxytocin (OT) has been measured in maternal and fetal blood. Simultaneous samples were obtained in maternal forearm venous blood and in umbilical venous and arterial blood in 29 patients at term delivery. In addition, maternal forearm venous blood samples were also obtained 10 minutes prior to delivery. Mean OT level in maternal plasma at delivery was 82 ± 12 μU/ml, and at 10 minutes prior to delivery the mean OT level was 90 ± 11 μU/ml. The umbilical arterial plasma OT showed 95 ± 12 μU/ml and the umbilical vein plasma OT was 60 ± 10 μU/ml. Oxytocin levels higher in maternal blood than in fetal blood were found with the following incidence: In 51% of samples there was more OT in maternal venous blood than in umbilical arterial blood, and in 84% of samples there was more OT in maternal blood than umbilical vein blood. During the postpartum period, the mean maternal plasma OT was 66 ± 8 μU/ml for the first day, and 50 ± 9 μU/ml and 54 ± 9 μU/ml for the second and third days, respectively. This study indicates that both the fetus and the mother are active producers of oxytocin.