Ultrastructural aspects of the absorption of nutrients by the rat small bowel under natural feeding and administration of a food mixture homogenate to a small bowel strip were studied. It is shown that as early as 25 min after the onset of feeding, nutrients get into the proximal part of the small bowel, run across the epithelial barrier and enter the stroma of the intestinal villi and the vascular bed. Active absorption of nutrients is also observed in situ under limited intracavitary hydrolysis. The identified nutrients penetrate the apical membrane of enterocytes by pinocytosis, are detectable but inside vesicular structures of the cell, and get into the intercellular space by exocytosis. The concept of the existence of the control systems preventing foreign substances from penetration in the body and eliminating the sequels of such a penetration has been worded.