The placental transfer and tissue distribution of tritium-labelled prostaglandin F lα was investigated in rats on day 14 of gestation. Low levels of radioactivity were detected in the embryos and placentas 5, 10, 60, and 120 minutes after administration of the prostaglandin, but the concentration in amniotic fluid was extremely high at all time intervals. Maternal serum, kidney, liver and lung revealed a greater uptake of the isotope compared to the uterus, ovary, spleen and brain. It is conceivable that the high levels of prostaglandin accumulated in the amniotic fluid may act directly on the conceptus.