The importance of the presence of food in the gastointestinal (GI) tract in maintaining normal enzyme activity in the small intestine was investigated in rats that were kept from eating for as long as 2 weeks. During this time animals were sustained on a nutrient solution administered intravenously. Enzyme activity and small bowel mass in these animals and similiar parameters in rats that were administered the same solution orally were compared with control groups fed an essentially isoenergetic diet of stock rat food and water. Disaccharidase and peroxidase levels in mucosal scrapings expressed as activity/milligram protein, activity/intestine, and activity/100 g body weight, were significantly reduced in intravenously nourished rats but not in rats receiving the solution orally. In addition to changes in enzyme activities, parenteral feeding was accompanied by a significant reduction in the small bowel weight:body weight ratio as compared with that of rats fed the stock diet or nutrient solution orally. Results support the conclusion that normal activity of intestinal enzymes spatially distributed with regard to depth within the mucosa depends on the presence of food in theGI tract and cannot be maintained at normal levels by total intravenous nutrition.