Abstract Oxidative stress occurs in diabetic patients and experimental models of diabetes. The ability of L-arginine to ameliorate oxidative stress after treatment with alloxan was investigated in rats. Adult male rats were injected intraperitoneally with multiple doses of alloxan to produce experimental oxidative stress characteristic of diabetes mellitus. The rats were maintained in this state for eight weeks. Rats were decapitated and gastrointestinal tissues were isolated. The results were interpreted and the significance was analyzed using unpaired, two-tailed Student’s t tests and two-way repeated measures ANOVA. Hyperglycemia was observed in the plasma after three days of alloxan treatment. This was associated with a depression of glutathione (GSH) concentration as well as superoxide dismutase (SOD) and catalase (CAT) activities in the pylorus and ileum. In addition, the malonyldialdehyde (MDA) levels were significantly elevated, indicating increased lipid peroxidation and oxidative stress in the same tissues. L-arginine supplementation (0.15mg/ml) through drinking water until eight weeks after alloxan injection significantly ameliorated the oxidative stress, as evidenced by lower MDA levels as well as higher levels of endogenous GSH, SOD, and CAT (p<0.001). These effects were paralleled by marked protection and prophylaxis against alloxan-induced hyperglycemia. Thus, exogenously administered L-arginine might improve the clinical manifestation of diabetes mellitus and decrease the oxidative stress in the gastrointestinal tract. In addition, the study supports the beneficial effects of L-arginine. These effects might be attributed to its direct, NO-dependent antioxidant capacity and/or NO-independent pathways.