The morphological changes in the brain tissue of Wistar rats of different ages after intramuscular insulin injection in the dose of 1 IU/kg are typical for the hypoxic lesions of nervous tissue. The brain of rats at the age of 1-2 months responds to increased deposition of glycogen, moderate swelling around the cells and blood vessels. The effect of insulin on the brain Mature rats at the age of 5-7 months leads to vasodilatation, more pronounced swelling around the cells and blood vessels, hypertrophy of cells, aggregation and diabetes of red blood cells. The severity of ischemic changes significantly increased in the brain of old rats at the age of 20-24 months. These old rats had the senile dendrites, the widespread hypertrophic degenerative changes, i.e. flask-shaped vasodilation, hyperemia. In most of the visual fields are detected capillaries with the presence of aggregation on the side erythrocytes, signs of micro thrombosis, hemorrhage areas. The authors note that there is a loss of tone and tortuosity of the small arterioles, widespread swelling around the cells and around the blood vessels. Morphological signs of brain reaction on insulin injection reflect the death of cortical neurons, marked swelling of the brain tissue, disruption of vascular permeability, the thrombus formation and hemorrhages.