The concentrations of plasma PGF2 alpha and its main metabolite, 13,14-dihydro-15-keto-PGF2 alpha (PGFM) were measured in serial samples of blood collected in 10 pregnant women at term who were given iv infusions of low doses of PGF2 alpha for induction of labor. Six other women served as controls and were given saline infusions. Uterine contractions began with a mean latency of 62 min in the PGF2 alpha infused women, in controls uterine activity remained unchanged. Plasma PGFM levels had increased significantly 30 min after PGF2 alpha infusion began, rising thereafter in a dose dependent manner. Plasma PGF2 alpha also rose reaching a steady state at 2 hours. No significant changes were observed in the controls. The 6-h infusion resulted in delivery in 5 of the 10 women, in the 5 others the cervical scores increased only by 1.25 points on the average and further treatment was needed to achieve delivery, although prostanoid levels rose to similar levels in all. The data show that when uterine contractions are induced by systemic PGF2 alpha, the levels of PGFM are significantly raised. In spontaneous labor uterine contractions begin long before plasma PGFM rises. Thus, if endogenous PGF2 alpha generation is involved in the initiation of uterine contractions during spontaneous labor, it must be synthetized in the myometrium at quantities too low to raise the levels of circulating PGFM.