In an attempt to obtain further information on the influence of the intestinal contents on the development of mucosal structure and function, self-emptying blind loops of rat jejunum were constructed, and the oral end was exteriorized with a stoma to permit infusion of different solutions into the loop. Ringer solution or media containing glucose or galactose was instilled into the loops three times daily for 27 days before functional and structural examination of the loops. The body weight and food intake of the animals did not differ significantly from one group to another. Treatment with glucose, but not with galactose or Ringer solution alone, induced a significant increase in the villus height. Disaccharidase but not dipeptidase activity was concomitantly increased. Infusion of glucose or galactose both led to an increase in the transport capacity of the loop, as determined by glucose absorption in vivo. These results indicate that glucose has a nutritive effect, probably due to its intracellular metabolism, which is responsible for the structural alterations observed. On the other hand, the increase in transport capacity may be attributed to substrate-induced stimulation of the specific monosaccharide transport system in the epithelial cell.