Prolonged overfeeding of rats with a highly caloric mixed food led in the majority of the animals to a more intensive increase in body weight and of fat depôt than in control animals given conventional laboratory diet. Rats with adiposity caused by prolonged overfeeding displayed disturbances of carbohydrate and lipid metabolism expressed in elevation of blood sugar level on fasting stomach, a fall of glucose tolerance and of insulin sensitivity, and also in increase of blood insulin activity and of free fatty acids and blood triglycerides on fasting stomach. The number of fat cells in the adipose tissue remained unchanged, but their volume increased. In the animals with adiposity the relative area of the islar tissue of the pancreas and the mass of beta-cells increased significantly; the count of the islar cells in the glandular parenchyma increased as well.