With light and electron microscopy structure and composition of the small intestine parietal mucous layer of rat, chicken, and man were studied. It has been shown that parietal layer is complicated and multicomponent system includes mucous glycoproteins, vesicular and membrane structures, epithelial cells fragments, food substrates particles and bacteria. It is assumed that subepithelial mucous layer components may be of significance in premembrane digestion and absorption.