Abstract: Objective To observe the effect of L-arginine on diabetic rats. Methods Forty adult male Lewis rats were randomized equally into diabetic and normal control groups, and the former rats were treated intraperitoneally with streptozotocin to induce diabetes mellitus. Seven days later, half of the diabetic and normal rats were injected intraperitoneally with L-arginine at the daily dose of 1 g/kg, while the remainder were given saline instead. All the rats were euthanized on 10 days after L-arginine or saline treatment, and their body weight, plasma protein, arginine and sugar, food and water intake were analyzed. Results Diabetic rats had obviously decreased body weight, plasma protein and arginine but increased blood sugar and food and water intakes in comparison with the control rats. L-arginine significantly increased plasma protein and arginine, decreased food and water intakes, but failed to prevent weight loss and blood sugar increment in diabetic rats as compared to their saline-treated counterparts. L-arginine supplementation did not result in any changes other than arginine elevation in the control rats. Conclusion L-arginine supplementation can partially improve polydipsia and polyphagia and increase plasma protein in diabetic rats.