A jejuno-ileal bypass operation which excluded about 85% of the small intestine was conducted in two groups of rats to evaluate the effects of oral feeding versus total parental nutrition (TPN). One group received food orally while another group received TPN. Two other groups of nutritive control animals underwent laparotomy and suture markings on the intestine. Animals were sacrificed two weeks after the shunt or sham operation. The liver, kidneys, pancreas and spleen were weighed. The wet and dry weights and the villus height of small intestinal segments were also determined. The shunt-operated TPN rats showed a body weight gain of 28% over the initial body weight while the orally fed animals showed an increase of 3.2%. The TPN controls had a negative nitrogen balance only on the day of the sham operation, whereas the TPN shunt-operated animals changed to positive nitrogen balance on the second postoperative day. The wet weights of the liver, kidneys and spleen were significantly higher in TPN animals. The functioning region of the small intestine in the shunt-operated animals was hypertrophic in both the orally fed and the TPN animals, but there were greater quantitative changes in the orally fed animals. The experiments demonstrated the beneficial effect in rats of TPN over oral nutrition for the treatment of surgically induced malabsorption. The studies also showed that the small intestine is capable of developing compensatory mucosal action without intraluminal nutritive content, although the presence of food in the lumen appears to stimulate morphological changes to a greater extent after massive small bowel resection.