p-Aminohippuric acid (PAH) uptake was studied in basal-lateral membrane vesicles prepared from rabbit renal cortex. An outwardly directed hydroxyl gradient (pHo = 6.0, pHi = 7.6) stimulated PAH uptake slightly over that when the internal and external pH values were equal at 7.6. A 100 mM sodium gluconate gradient directed into the basal-lateral membrane vesicles increased PAH uptake about 2-fold over that when N-methyl-D-glucamine or potassium gluconate gradients were present. When hydroxyl and sodium gradients were simultaneously imposed (pHo = 6.0, pHi = 7.6 and 100 mM sodium gluconate extravesicularly) PAH uptake was stimulated greater than with the pH or Na+ gradient alone. In fact, an 'overshoot' was observed. Countertransport experiments showed that either intravesicular PAH or intravesicular PAH and Na+ could stimulate 3H-PAH uptake. Probenecid, an inhibitor of organic anion transport, inhibited both the hydroxyl-stimulated and Na+ gradient-stimulated PAH uptake but the greatest inhibition by probenecid was seen when the hydroxyl and sodium gradients were both present. Thus, it is proposed that the driving force for PAH accumulation across the basal-lateral membrane of the proximal tubule is a transport system which moves Na+ and PAH into the cell for an hydroxyl ion leaving the cell, i.e. a sodium-dependent anion-anion exchange system.