In order to investigate mechanisms of fetal-maternal (F-M) transfer of Na + , clearance of 2 2 Na + and 5 1 Cr-EDTA was measured simultaneously across the dually perfused placenta of the rat. In eight experiments clearance was measured successively in the F-M (K f m ) and in the maternal fetal (M-F; K m f ) directions. Clearance of 2 2 Na + in the two directions was approximately equal (K m f = 11.6 +/- 2.0 μl/ min; K f m = 11.1 +/- 1.7 μl/min: mean +/- s.d.) while K f m of 5 1 Cr-EDTA (4.4 +/- 0.7 μl/min) was nearly double K m f (2.4 +/- 0.8 μl/min) for this tracer. Even greater asymmetry in the transfer of 5 1 Cr-EDTA was found when measured across intact (non perfused) placenta. It is suggested that this asymmetry is caused by volume flow in the F-M direction. In other experiments transfer was measured in the F-M direction only. Ouabain (0.1 mM) on the maternal side and reduced concentration of Na + (25 mM) on the fetal side had no effect on the F-M transfer of the tracers. Reducing the temperature of the preparation by 5 o C significantly decreased transfer of 2 2 Na + . The transfer of 2 2 Na was inversely related to the concentration of K + on the fetal side. These observations suggest that the F-M transfer of Na + has three components: diffusion through paracellular routes; convective flow by filtration through wide placental pores, and transcellular transport by a mechanism which is uncertain at present.