To determine the role of sulfhydryl group in transport of organic ions across the basolateral membrane of renal proximal tubules, effect of p-chloromercuribenzoic acid (PCMB) on the transport of tetraethylammonium (TEA) and p-aminohippurate (PAH) was studied in rabbit renal cortical slices. PCMB caused irreversible inhibition of TEA and PAH uptake in a dose-dependent manner, with I₅₀ value (concentration for 50% inhibition) of 30 μM for TEA and 75 μM for PAH. Kinetic analysis of TEA and PAH uptakes showed that PCMB decreased Vmax (62.35 vs. 28.32 n mole/g·min fur TEA: 385.24 vs. 170.36 n mole/g·min for PAH) without changing Km. The inhibitory action of PCMB on TEA and PAH uptakes was independent of pH of the pretreatment medium. The inhibitory effect of PCMB on the uptake of TEA or PAH was prevented by dithiothreitol, but not by the substrate. PCMB inhibited Na-K-ATPase activity in a dose-dependent manner with I₅₀ value of 50 μM, which is similar to those for TEA and PAH uptake. These results suggest that PCMB inhibits the transport of organic cations and anions in the renal basolateral membrane by directly affecting the SH-group in the transporter molecules or secondly by altering the Na-K-ATPase activity.