Effects of anions on p-Aminohippurate (PAH) transport across the basolateral membrane (BLM) were studied. Basolateral membrane vesicles were isolated from rabbit renal cortex by using a self-orienting Percoll-gradient centrifugation and Mg²⁺-precipitation. The characteristics of the membrane vesicles was examined by marker enzyme activity, membrane orientation and transport studies. The Na-K-ATPase activity in the fraction containing BLM vesicles was enriched 9¡¤fold, and the alkaline phosphatase activity in the fraction containing BBM vesicles was increased 9-fold, compared with those of the homogenate. The transport properties of the two membrane preparations were studied by a rapid filtration technique. The uptake of PAH by BLM was sensitive to changes in medium osmolarity and inhibited by probenecid. When the uptake of 50μM PAH in voltage-clamped BLM vesicles was determined in the presence of various anions in the incubation medium, cis inhibitions by SO₄ and SSO₃ were observed in the presence of sodium gradient (out>in). Sodium-dependent PAH uptake was inhibited competitively by external SO₄ PAH uptake in BLM vesicles loaded with 20 mM acetate and SO₄ or 200 μM PAH was significantly stimulated as compared with unloaded vesicles. The extent of trans-stmulation of PAH uptake by SO₄. was increased with the inside concentration of SO₄. This trans-stimulatory effect by SO₄, was observed to be additive in the presence of Na gradient and completely inhibited by 2 mM probenecid and 1 mM SITS. These results demonstrate that PAH/anion exchange is present in BLM of renal cortex and in this exchange mechanism inorganic as well as organic anions are involved as substrates.