Transport mechanisms of p-aminohippurate (PAH) were investigated in rat renal brush-border membrane vesicles. The uptake of PAH was stimulated by an inside-positive membrane potential created by K+ and valinomycin. This potential-stimulated uptake of PAH was inhibited by various anion transport inhibitors and was saturable. In addition, PAH uptake in the presence of valinomycin was linearly increased in proportion to log[K+]out/[K+]in. On one hand, PAH uptake was stimulated by [14C]PAH/PAH or [14C]PAH/Cl- exchange, and the [14C]PAH/PAH exchange was insensitive to the membrane potential. The uptake by the exchanger was also inhibited by anion transport inhibitors, but the potential-stimulated uptake of PAH was more sensitive to furosemide and 4,4'-diisothiocyano-2,2'-disulfonic stilbene. On the contrary, [14C]PAH/PAH exchange was more sensitive to urate than the potential-stimulated uptake of PAH. These findings indicate that PAH is transported by two distinct transport systems in rat renal brush-border membranes, a potential-sensitive transport system and an anion exchanger.