Plasma prostaglandin F2alpha (PGF2alpha) concentrations were determined by radioimmunoassay in serial samples during labor and the early postpartum period. PGF2alpha was also measured during and between contractions every 20 to 30 seconds. The highest levels of PGF2alpha were found during expulsion of the fetus and placenta with an additional rise half an hour after delivery and a decrease 2 hours after delivery to the levels of first stage of labor. PGF2alpha values from cord blood were higher than in maternal peripheral blood in labor. The maximum PGF2alpha concentration occurred between 100 to 120 seconds after the peak uterine contraction and between 40 and 60 seconds prior to peak of the next contraction. The possible explanation is that PGF2alpha initiates uterine contractions and plays a role in the maintenance of labor, in the expulsion of the fetus and the placenta, and in the involution after delivery.