The article studies the features of the process of breast milk absorption in the early postnatal period. The material of the study was outbred white rats, which were on different types of feeding. The process of absorption in the small intestine is universal and proceeds almost identically up to 14 days after birth when breastfeeding. In mammals and humans, the mucous membrane of the small intestine and its immune system begin to form before birth, and their final formation occurs after birth, by the time of transition to final nutrition [1,2]. The immune system of the intestine at the time of birth is presented in the form of clusters of mesenchymal cells under the epithelium, from which the reticular stroma is formed at the beginning, and then single lymphocytes and their precursors from the vascular bed are populated in it. The lymphoid tissue of the mucous membrane of the small intestine in this period does not have the characteristic features for a sexually mature organism [3,4,8,11,17].