In the present study, kidney superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), catalase (CAT) activities, vitamin C and lipid peroxidation levels were investigated in diabetic rats. Diabetes was induced in rats by streptozotocin and the treated rats received 1 g/l vitamin C with 0.5 mM CoCl2 in drinking water at 2-week intervals for 6 weeks. Kidney SOD, GSH-Px, CAT activities and lipid peroxidation levels were significantly increased in diabetic rats at the end of the 2nd, 4th and 6th weeks (p < 0.05), whereas vitamin C level was decreased significantly (p < 0.05) at the end of the 6th week compared to those of controls. Vitamin C with cobalt treatment of diabetic rats resulted in partial restoration of SOD and CAT activities, thiobarbituric acid reactant substances and vitamin C levels at all times studied, whereas treatment did not change GSH-Px activity. These results suggest that vitamin C with cobalt effectively normalized hyperglycemia (at the end of the 6th week) but could not completely restore the altered endogenous defence systems in diabetic rat kidney.