Plasma oxytocin levels in the umbilical artery (UA) exceeded umbilical venous levels in newborn infants delivered by cesarean section (without maternal labor) (p less than 0.05) and following labor (NS). There was an initial rapid decrease in oxytocin concentration from UA levels to those in peripheral venous blood by 30 min of page. Plasma oxytocin levels for breast-fed and formula-fed infants remained elevated over adult basal levels (1.7 +/- 0.3 mu U/ml) throughout the 4-day study period. Mean oxytocin concentration measured in breast milk from 10 mothers 2-4 days following vaginal delivery was 10.0 +/- mu U/ml. The stimulus for fetal and neonatal oxytocin secretion remains obscure, but continues beyond the period of birth.