The article is devoted to studying the formation of duodenal endocrine cells in the early postnatal period. The material of the study was outbred white rats, which were on different types of feeding. The small intestine with normal microflora affects the adequate adaptation of functional systems of various levels of organization, metabolism, normal structure and function of all organs and systems, the body as a whole. It is known that natural, mixed or artificial, definitive nutrition involves the intake of nutrients and a huge number of various microorganisms from the external environment into the intestine. As a result, during the formation of a normal intestinal microbiocenosis, a genetically determined harmonious development of the individual is ensured.