The uptake of sodium into vesicles isolated from the microvillous, maternal-facing plasma membrane of human placenta was studied. In equilibrium exchange conditions, sodium entry increased with time towards an equilibrium value after 30-60 min. Over 90% of the uptake was into an osmotically active space and the initial rate of uptake was halved by amiloride but unaffected by loop diuretics and capnophorin inhibitors. The apparent inhibition constant (Ki) for amiloride was 3.5 x 10(-6) M and the Michaelis constant (Km) with respect to sodium of the amiloride-sensitive component was 7-11 mM. With an imposed outward H+ gradient, sodium was transiently accumulated within the vesicles. The overshoot was abolished by amiloride and shown, by experiments with FCCP (carbonyl cyanide p-trifluoromethoxyphenylhydrazone) and potassium and valonomycin, not to be dependent on any electrical potential generated by the H+ gradient. The evidence for Na+-H+ exchange at this surface and its possible functions are discussed.