Measurements of oxytocin in maternal and fetal circulation during human labor are reviewed and related to known changes in uterine oxytocin sensitivity during pregnancy and labor. It is concluded that oxytocin is secreted in short-lasting spurts; therefore levels measured with infrequent intervals do not give adequate information on the amounts of oxytocin secreted during labor. Presently, there is little evidence for an increased maternal secretion rate of oxytocin at the onset of labor, but during labor a progressive increase occurs, with a maximum at the expulsive phase. Fetal secretion rate also increases markedly during labor, but the timing of this increase is still unknown. The dramatic increase in human uterine oxytocin receptors at term makes the uterus responsive to very small amounts of oxytocin. Hence, an increased oxytocin secretion rate is not a necessary prerequisite for oxytocin-stimulated contractions during labor. Evidence from oxytocin receptor blockade and suppression of oxytocin secretion supports the concept that oxytocin is an important stimulus for uterine contractions in early human labor.