The protective effect of N-(2-mercaptopropionyl)-glycine (tiopronin), a clinically used sulfhydryl-containing compound, on cisplatin-induced toxicity to rat renal cortical slices was investigated. Exposure of the slices to cisplatin (2 mM) resulted in toxicity, as shown by an increase in leakage of the two enzymes aspartate aminotransferase and lactate dehydrogenase into the incubation medium and a time-dependent decrease in the reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) by the slices. Tiopronin (2 mM) completely prevented the cisplatin-induced increase in enzyme leakage and substantially blocked the decrease of MTT reduction caused by cisplatin. These protective effects were concentration-dependent and furthermore, the depletion of ATP, glutathione and induction of lipid peroxidation in the slices by cisplatin (2 mM) were reversed by 2 mM tiopronin. Pretreatment of slices with tiopronin for 60 min also significantly protected the renal slices from cisplatin-induced toxic effects. These protective effects, however, were abolished by p-aminohippuric acid, a compound with some structural similarity to tiopronin, which both undergoes and inhibits active transport in the cells of the proximal convoluted tubule. Cisplatin (1 mM) also depleted the free sulfhydryls of tiopronin (1 mM) in a second incubation medium system and PAH (2 mM) diminished the extent of this depletion somewhat. These observations suggest that tiopronin protects against cisplatin-induced nephrotoxicity by acting as an alternative target for cisplatin both intra- and extracellularly and thus protects against cisplatin-induced depletion of glutathione in the kidney cell.