Using "high performance" liquid chromatography, we studied non-protein-bound fractions and total concentrations of 18 solutes accumulating in sera from a group of 12 patients who were undergoing chronic ambulatory peritoneal dialysis (CAPD) and in predialysis sera from a group of 15 hemodialysis (HD) patients. We monitored longitudinal changes in solute concentrations for two patients with respect to change of therapy between HD and CAPD. The concentrations of pseudouridine (P less than 0.001), uric acid (P less than 0.001), and an unknown fluorescent solute, "UKF3" (P less than 0.01), differed in sera of HD and CAPD patients. When standardized with respect to serum creatinine concentrations, the concentration of the transfer-RNA catabolite, pseudouridine, was significantly (P less than 0.0001) higher in sera of CAPD patients than in HD patients, suggesting an increase in turnover of transfer RNA. In stepwise linear discriminant analysis, the combination of pseudouridine and the probably biochemically related fluorescent unknown, UKF3, contributed most to the differentiation between sera from CAPD and HD patients.