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It is now known that rapid placental and fetal development is associated with elevated levels of circulating high density lipoprotein (HDL) in pregnant women. The main structure implicated in the maternal–fetal exchange is the syncytiotrophoblast, composed of a brush border membrane (BBM), facing the mother, and a basal plasma membrane (BPM), facing the fetus. In order to understand the mechanisms controlling the placental and fetal supplies of cholesterol, we purified both BBM and BPM and verified the presence of HDL binding sites in these membranes. Binding studies using¹²⁵I-HDL₃show a single affinity binding site on BPM which has a dissociation constant (K_d) of 3.45±0.43μg protein/ml and a maximal binding capacity (B_max) of 5.46±1.69μg protein/mg membrane proteins. In BBM, we observed two affinity binding sites, one with a K_dof 0.62±0.03μg protein/ml and another one with a K_dof 6.57±0.87μg …