The effect of the synthetic compound 2-morpholino-5-phenyl-6H-1,3,4-thiadiazine hydrochloride (L17) on rats with a streptozotocin-nicotinamide model of type 2 diabetes mellitus (DM2) was studied. The experiment was performed on 50 male Wistar rats. L17 was administered intramuscularly to diabetic rats at a dose of 40 mg/kg. Modeling DM2 revealed hyperglycemia, accumulation of glycated hemoglobin (HbA1c) and malondialdehyde (MDA); a decreased content of thiols; and compensatory activation of antioxidant defense (AOD) enzymes [superoxide dismutase (SOD), catalase, glutathione peroxidase] in erythrocytes. Administration of L17 to diabetic rats was accompanied by a decrease in hyperglycemia, normalization of HbA1c and MDA levels and AOD enzyme activity, and prevention of thiol depletion. Correction of hyperglycemia and imbalance in the free-radical oxidation(AOD system in the experiment made further study of the mechanism of the antidiabetic action of L17 promising.