Thiol compounds such as glutathione, homocysteine, and cysteinyl-glycine are the natural reservoir of reductive capacity of the cells. Chronic renal failure is accompanied by disturbances in redox status of plasma thiols. The aim of the present study was to compare the changes in concentrations of different forms of thiols in plasma of terminal renal failure patients, nondialyzed and on peritoneal dialysis. Total concentrations of different redox forms of thiols were determined by high performance liquid chromatography. We observed that total concentration of glutathione in terminal renal failure patients decreased and total concentration of the remaining thiols in these patients significantly increased. Continuous ambulatory peritoneal dialysis had the following features in comparison with nondialyzed patients: (1) glutathione and cysteine concentration was restored and (2) free fraction of thiols rose, while protein-bound fraction dropped (except for homocysteine). Continuous ambulatory peritoneal dialysis corrects total concentration of glutathione and cysteine, in comparison with nondialyzed patients.