The aim of our study was to evaluate the levels of nitrite in plasma and methemoglobin in whole blood using electron paramagnetic resonance (EPR) in patients with type 2 diabetes mellitus receiving metformin. After 3 months of continuous use of the drug at a dose of 1700 mg in parallel with the improvement of carbohydrate and lipid metabolism, reduction in waist circumference there was a statistically significant increase in nitrite concentration in plasma and increased methemoglobin in whole blood. Increased bioavailability of nitric oxide during treatment with metformin may be an additional explanation for the mechanism of drug action.