1. This study was conducted on 467 cases of non-insulin-dependent diabetes mellitus and 180 healthy controls. Lipid peroxidation products in plasma and erythrocytes were assayed as thiobarbituric acid reactive substances, along with the erythrocyte antioxidant enzymes, namely superoxide dismutase, catalase and glutathione peroxidase. In addition, scavenger vitamins A, C and E and reduced glutathione levels in blood were also measured. 2. Lipid peroxidation was significantly raised within the first 2 years of diagnosis, and superoxide dismutase, catalase, reduced glutathione and vitamins C and E were significantly lowered. 3. These changes were correlated with the duration of the disease and were of a higher magnitude with the development of complications. 4. The results suggest that the antioxidant deficiency and excessive peroxide-mediated damage may appear early on in non-insulin-dependent diabetes mellitus, before the development of secondary complications.