The effect of pH on the rate of phosphate (Pi) uptake was studied in rabbit duodenal brush-border membrane vesicles. Pi uptake was found to be sodium dependent at all pH values tested (5.7-8.1). Further, the rate of Pi uptake depended on pH; for instance, with 100 mM external sodium, reducing the pH from 8.1 to 6.8 or 5.7 doubled the rate of Pi influx. At 100 mM external sodium, experiments under initial rate conditions, carried out with varying Pi concentrations and at pH values of 6, 6.8, or 7.6, showed that sodium-dependent Pi uptake was saturable at the three pH values tested; the apparent Km expressed in function of total Pi was not dependent on pH. Vmax was not affected between pH 6 and 6.8 but was significantly reduced at pH 7.6. Lowering external sodium lowered Vmax at all pH values investigated. At acid and alkaline pH the rate of Pi uptake was a sigmoidal function of the external sodium concentration. Hill coefficients, calculated from these experiments, exceeded unity and were unaffected by pH. At saturating sodium concentrations, the rate of Pi uptake was higher at pH 6 than at pH 7.6. The [Na]0.5 was lower at pH 7.6 than at pH 6. Further, sodium-dependent Pi uptake appeared to be electrogenic at acid and alkaline pH. It is concluded that the pH dependence of intestinal Pi transport is not an expression of preferential transport of monovalent or divalent phosphate. The pH dependence appears to reflect properties of the sodium-phosphate cotransport mechanism and is in part related to changes in the affinity of the transport system for sodium.