OBJECTIVE: In this study, it was aimed to investigate the antioxidant protective effect of melatonin supplementation against oxidative damage in blood and liver tissues of rats with experimental type 1 diabetes with Streptozotocin (STZ). MATERIALS AND METHODS: Forty adult Spraque Dawley male rats; Group 1-Control group, Group 2-diabetes group, Group 3- melatonin supplement group and Group 4- diabetes and melatonin supplement group were divided into four groups. Diabetes was induced in rats by administering a single dose of 60 mg / kg STZ intraperitoneally. Melatonin supplementation was administered at a dose of 10 mg / kg / day subcutaneously for 6 weeks. Glutathione (GSH), catalase (CAT) and malondialdehyde (MDA) levels were measured spectrophotometrically in the liver and serum samples taken at the end of the experiment. RESULTS: The results of the study showed that MDA levels increased in the serum and liver tissues of diabetic animals. It was observed that melatonin administration decreased MDA levels in both tissues, increased GSH and CAT levels or kept them stable by preventing it from falling (p <0.05). CONCLUSION: The results of this study are consistent with the consensus that emphasizes and accepts that one of the main causes of complications of diabetes is oxidative stress. Our results show that melatonin has an ameliorating effect on increased oxidative stress in diabetes and melatonin can be used as a therapeutic agent.