The aim of this work is to study the membrane hydrolysis of sucrose and the absorption of glucose and fructose formed in the small intestine of rats under the influence of diets with different carbohydrate content in early postnatal ontogenesis. The studies were performed on male Wistar rats. After weaning, the rat pups of the experimental groups were kept for 6 weeks on diets with a high (78.2% of energy intake) or low (27.9% of energy intake) content of carbohydrates, or on a carbohydrate-free diet, then a part of the animals of each of the experimental groups were kept on a standard diet for 3 days, 2 or 6 weeks. Animals kept after weaning on a standard diet served as controls. The method of perfusion of an isolated loop of the small intestine in situ was used to study the digestive and transport processes. It was found that a high carbohydrate diet leads to an increase in the hydrolysis of sucrose and the absorption of the resulting glucose and fructose (by 1.2–1.6 times) depending on the initial concentration of sucrose (12.5; 25 and 37.5 mM). Under the conditions of a low-carbohydrate diet, the studied parameters do not change at low concentrations of sucrose (12.5 and 25 mM), and at higher concentrations they decrease by 1.2-1.4 times. As a result of a carbohydrate-free diet, there is a sharp decrease in sucrose hydrolysis and glucose absorption (by 3 or more times) and fructose absorption is practically leveled. The transfer of animals from a low-carbohydrate diet to a standard diet leads to normalization of monosaccharide absorption after 3 days, and membrane hydrolysis of sucrose after 2 weeks. The transfer of animals from a high-carbohydrate diet to a standard diet leads only to a partial normalization of the intensity of hydrolysis and transport processes even after 6 weeks. The transfer of animals from a carbohydrate-free diet to a standard diet causes a gradual increase in membrane hydrolysis of sucrose and absorption of monosaccharides, however, even after 6 weeks, the absorption rate remains 1.5 times lower (glucose) and 3.5 times (fructose) than in the control. Thus, long-term high-carbohydrate and carbohydrate-free diets in early postnatal ontogenesis, in contrast to a low-carbohydrate diet, contribute to the development of disorders in the membrane hydrolysis of sucrose and absorption of glucose and fructose formed in the small intestine.