Gain in body mass, mucosal morphology and microflora of the contents of the cecum were experimentally studied in adult growing male rats kept for 15 days on parenteral (intravenous) feeding with various nutrient mixtures. The rats on parenteral feeding and the control ones fed orally (via the natural route), kept on diets including all the major nutrients, gained in body mass about 40 g, during the experiment. Body mass of the animals on parenteral feeding with mixtures not containing the nitrous component was practically unchanged by the end of the experiment in all the groups of animals. Dystrophic changes in the cecum were detected in rats kept completely on parenteral feeding. These changes manifested mainly in altered nuclear structure of cryptal epithelium, being especially grave, up to the death of cells, in cases of reduced depth of the cryptae, in rats fed diets free from nitrous components. In rats parenterally fed diets including the principal nutrients the level of anaerobic Lactobacilli in the cecum was significantly reduced, as compared to rats on natural feeding.