In this study we have investigated the oxidative metabolism of red blood cells (RBC), plasma, serum, aqueous humor, and lens of healthy subjects and of age-matched cataractous patients with and without diabetes. Reduced and oxidized glutathione (GSH GSSG) levels in RBC were similar among the three groups. Plasma levels of GSSG were higher in diabetics than in cataractous and control subjects. No differences in plasma content of GSH were noted among the three groups. The activity of the enzyme glucose-6-phosphate dehydrogenase was significantly diminished in diabetic patients. Controls and cataractous patients showed similar levels of malondialdehyde (MDA). Although not significant the MDA content in RBC from diabetics was elevated. No differences in plasma levels of vitamin E were noted among the three groups. The biological liquid oxidant activity of serum in diabetic patients was significantly higher than in controls and cataractous patients. GSH levels in aqueous humor were similar in diabetic and nondiabetic cataractous patients. The content of GSSG in aqueous humor was highest in diabetic patients. Control clear lenses showed low levels of MDA. The MDA levels in cataractous lenses from nondiabetic patients were significantly higher than those of controls. In diabetic patients the content of MDA in the lens was approximately twice as high as the cataractous values. Our results seem to demonstrate that oxidative damage could play a role in the pathogenesis of cataract in diabetes.