Serial radioimmunoassay measurements of plasma oxytocin (OT) in maternal venouse blood were carried out in 15 patients, during pregnancy, labor and delivery. Fetal plasma OT was also measured in the blood from the umbilical vein and umbilical artery. During pregnancy, plasma OT is present in maternal blood. The quantity of plasma OT increases with advancing pregnancy. No OT surge occurs around the onset of labor. The plasma OT surge occurs with cervical dilatation and vaginal distention. OT does not play a primary role in the initiation of labor. OT most likely contributes to formation of prostaglandins through the uterine contractions OT produces. OT surges, frequently called spikes, have been observed to occur during pregnancy as well as during labor. During the course of labor, OT surges have been encountered in association with rupture of the membranes, vaginal examination and descent of the vertex, and have occurred almost consistently with maximal cervical and vaginal distention. Such OT surge was suppressed by effective spinal and pelvic regional anesthesia. This surge is consistent with the Ferguson reflex described in experimental animals, and it represents the 1st evidence that the Ferguson reflex, in fact, exists in humans. Evidence is presented here that an excess of OT in fetal blood over that found in maternal plasma was associated with hypertonic, irregular, tumultuous or prolonged labor and with mild to moderate fetal hypoxia and fetal distress peculiar to abnormal uterine contractions.