The aim was to determine the influence of melatonin on basal levels of glucose (BG), the level of thiobarbituric acid reactive compounds (TBCRC), reduced glutathione (GSH) levels, activities of glutathione reductase (GR), glutathione peroxidase (GPx), glucose-6-phosphate dehydrogenase (G-6-PhD) in the heart tissue of alloxan diabetic rats. Alloxan diabetes was evoked via intraperitoneal injection with a 5% solution of alloxan monohydrate in a dose of 170 mg/kg of body weight. There were 5 groups: 1) control group (n=8); 2) group with DM (BG level ≥ 8.0 mmol/l, n=8); 3) alloxan diabetic animals with DM which were injected with melatonin (n=8); 4) alloxan diabetic rats with impaired glucose tolerance (IGT) (BG level ≤ 6.9 mmol/l, n=8); 5) alloxan diabetic animals with IGT which were injected with melatonin (n=8). Melatonin ("Sigma", USA) was injected intraperitoneally in a dose of 10 mg/kg of body weight at 8 a. m. daily over 7 days starting with a 5th 24-hour period after the alloxan injection. The level of TBCRC increases by 70% in the heart tissue of alloxan diabetic rats with DM compared with the control value, whereas the G-SH, the activity of GPx, G-6-PhD and GR decreased by 46%, 18%, 39% and 21% respectively. The introduction of melatonin to alloxan diabetic rats is conducive to a decrease in them of the level of BG, as well as - a stabilization of the indices of the body's antioxidant defense such as activities of GR, GPx, G-6-PhD, content of TBCRC and G-SH in rats' heart.