The present study was designed to investigate the origin of prostanoids that are present in amniotic fluid. We evaluated the concentration of prostaglandins in fetal urine that was obtained from newborn infants immediately after delivery either before or after the onset of spontaneous labor or after oxytocin-induced labor. We found that the levels of prostaglandin (PG) E2, PGF2 alpha, 6-keto-PGF1 alpha, and 13, 14-dihydro-15-keto-PGF2 alpha in the fetal urine of newborn infants delivered after the spontaneous onset of labor and after oxytocin-induced labor were significantly greater than the levels in the fetal urine of newborn infants delivered before the spontaneous onset of labor. We conclude that one source of the prostanoids present in amniotic fluid is fetal urine, and, thence, fetal kidneys, and that labor, whether spontaneous or induced, caused increased excretion of prostanoids in fetal urine.