Oxytocin was measured in maternal and fetal plasma, amniotic fluid and neonatal plasma and urine using a specific radioimmunoassay, following extraction procedures with Florisil. Maternal oxytocin levels rose progressively with advancing gestation, but there were no significant differences between oxytocin levels around the onset of labor. No diurnal rhythm of oxytocin was evident in maternal plasma during the third trimester. Maternal and umbilical plasma oxytocin levels at spontaneous delivery were significantly higher than those at elective cesarean section. Maternal oxytocin levels in four cases of post-term delivery were lower than those during normal late pregnancy; all four cases experienced uterine inertia. All amniotic fluid samples had detectable oxytocin levels and there were no significant differences between oxytocin levels in the second trimester and those in the third trimester. Oxytocin levels in neonatal urine were higher than levels in amniotic fluid and lower than in the umbilical artery. Neonatal plasma oxytocin levels gradually decreased and oxytocin levels of 7-day-old infants were significantly lower than those in the umbilical artery, but higher than those in adults. In conclusion, it appears that maternal oxytocin levels may not be involved in triggering the onset of labor but may play a role in the maintenance and reinforcement of labor.