In basal-lateral membranes of the renal cortex, thiol substituted analogs of salicylate inhibited the uptake of p-aminohippurate (PAH). The mercury-containing analogs, thimerosal and mercaptide V (formed from 2 mol of thiosalicylate and 1 mol of Hg++), were highly inhibitory. Compared with probenecid, thimerosal and mercaptide V yielded dose-response curves of steeper slope and higher maximal effect. The dose-response curves of thimerosal and mercaptide V were similar in shape, although mercaptide V was more potent. The inhibition by thimerosal, mercaptide V and mersalyl acid, an anionic sulfhydryl reagent which also inhibits uptake of PAH ( Tse et al., J. Pharmacol. Exp. Ther. 226: 19-26, 1983), was found to be competitive with Ki values of 0.39 +/- 0.05, 0.12 +/- 0.06 and 0.05 +/- 0.02 mM, respectively. The inhibitory effects of thimerosal and mercaptide V were only partially reversible. Thimerosal and mercaptide V reacted 35 and 62%, respectively, with membrane sulfhydryl groups which may explain the nonreversibility of the inhibitor. The nonreversibility is probably not due to irreversible destruction of the vesicles, as the "glucose space" of the vesicles was not affected by these two compounds. That derivatives of salicylates capable of reacting with sulfhydryl groups were more inhibitory than thiosalicylate is consistent with a hypothesis that the PAH transporter contains a sulfhydryl group(s) essential for uptake. Based on the degree of inhibition, the degree of reversibility, the degree of membrane sulfhydryl group oxidation and the computer-generated three dimensional models of thimerosal, mercaptide V and mersalyl acid, a model of the PAH transporter is proposed.