Oxytocin levels in plasma and amniotic fluid were measured by a specific radioimmunoassay with a sensitivity of 5 pg/assay tube. Twenty-one of 28 pregnant patients had detectable levels of oxytocin ranging from 2.7 to 24.0 pg/ml. In 11 subjects in labor the mean ± SE of maternal venous plasma oxytocin was a) 43.1 ± 11.2 pg/ml (range: undetectable-72.5 pg/ml) in the first stage, b) 114.1 ± 20.7 pg/ml (range: 18.8–140.0 pg/ml) in the second stage, and c) 63.9 ± 10.8 pg/ml (range: 3.9–117.5 pg/ml) in the third stage. In 24 patients with normal labor and vaginal delivery, mean ± SE oxytocin concentrations were as follows: maternal venous plasma 40.6 ± 11.6 pg/ml (range: undetectable-72.5 pg/ml), umbilical arterial plasma 114.8 ± 18.3 pg/ml (range: 14.0–350 pg/ml), and umbilical venous plasma 37.5 ± 5.5 pg/ml (range: 6.3–107.5 pg/ml). Similarly, in 11 patients who had cesarean section after the onset of labor, maternal plasma oxytocin was 51.7 ± 13.1 pg/ml (range: 45.5–151.7 pg/ml); umbilical arterial plasma was 121.3 ± 29.9 pg/ml (range: 34.0–344.8 pg/ml); umbilical venous plasma oxytocin was 36.6 ± 10.1 pg/ml (range: 11.0–25.0 pg/ml); and amniotic fluid oxytocin was 43.5 ± 12.5 pg/ml (range: 11.0–96.0 pg/ml). In contrast, in 10 patients with cesarean section without labor, oxytocin level in maternal plasma was 26.7 ± 7.8 pg/ml (range: undetectable-63.5 pg/ml); in umbilical arterial plasma was 47.1 ± 7.9 pg/ml (range: 1.2–77.1 pg/ml); and in umbilical venous plasma was 27.6 ± 6.8 pg/ml (range: undetectable-60.3 pg/ml). The umbilical arteriovenous difference in oxytocin concentration was significantly higher in patients with labor than in patients without labor (P< 0.01). The results indicated 1) that maternal plasma has detectable levels of oxytocin during pregnancy which increases during labor with peak levels during the second stage, 2) a higher fetal concentration of oxytocin which flows towards the maternal compartment, and 3) that the fetus is a possible source of oxytocin for the mother during human labor.