Human placental brush border membranes were obtained from full-term normal vaginal deliveries. A sodium dependent transport system for phosphate was identified in these membranes. In the presence of a sodium gradient, at pH 7, T degree 35 degrees C, the apparent Km and Vmax were 86.5 +/- 24 microM and 1.9 +/- 0.18 nmol mg-1 30 s-1. The apparent Km was pH dependent, increasing to 500 +/- 55 microM when the pH was elevated to 8.5. At low substrate concentration, the optimal uptake was obtained at pH 7.0. Both monovalent and divalent forms of phosphate were transported so that alteration in the transport by pH is believed to be the result of a direct action on the transport system. Finally, sodium levels in the incubation medium dramatically influenced phosphate uptake by increasing the maximal velocity without modifying the affinity for the substrate. This active transport through the placental brush border membranes is believed to be at the origin of the maternal-fetal gradient of phosphate observed during pregnancy.