A new metabolite of naproxen, 6-desmethylnaproxen sulfate (6-DMNS), has been identified in plasma from normal and uremic subjects after oral administration of a single 500 mg dose of naproxen. Tentative identification was achieved by the finding of an increase in the concentration of 6-DMN upon incubation of the plasma with arylsulfatase from Helix pomatia or from Aerobacter aerogenes. More definitive identification was established through demonstration that the HPLC retention time of the conjugate is identical to that of an authentic reference sample of 6-DMNS. Unequivocal identification was accomplished by means of LC-MS after the metabolite was isolated from the plasma by protein precipitation with acetonitrile and further purified by anion-exchange and reversed phase HPLC. Plasma profiles of 6-DMNS for normal and uremic subjects, obtained by a procedure involving differential enzymatic hydrolysis using arylsulfatase from H. pomatia, revealed that 6-DMNS was present in plasma from all subjects but in relatively high concentrations only in subjects with impaired renal function and that the extent of the conjugation is related directly to the severity of the dysfunction. No evidence was found for the presence of glucuronide or sulfate conjugates of naproxen in plasma.