(3H) PGE2 uptake and transfer in the isolated perfused human placental cotyledon was assessed by a single pass paired isotope dilution technique utilising (14C) sucrose as an extracellular marker. Metabolism of (3H) PGE2 was measured by analysing maternal and fetal effluents from perfused human placental cotyledons after bolus injection of (3H) PGE2 into either the maternal or fetal sides. Maximal uptake of (3H) PGE2 was greater on the maternal (81 +/- 8%) than the fetal sides (42 +/- 12%) and showed saturation with increasing concentrations of PGE2 only on the fetal side with an apparent Km of 12 +/- 4.9 nmol/l and vmax of 1.5 +/- 0.2 pmol/min/g. Total recoveries of (3H) PGE2 were 84.6 +/- 11.8% and 32.6 +/- 6.3% of the injected dose after injection on the fetal and maternal sides respectively. Transfer of (3H) PGE2 was the same in both directions being 6.4 +/- 1.2% of the injected dose in the fetal-maternal direction and 5.8 +/- 2.7% of the injected dose in the maternal-fetal direction. Metabolism was greater on the maternal side (35% of injected (3H) PGE2) than the fetal side (18% of injected (3H) PGE2) and was principally to the 13,14-dihydro-15-keto-PGE2 metabolite. Metabolism of (3H) PGE2 after passage across the placenta was the same in both directions and was of the order of approximately 60%.