Three groups of rats were fed, respectively, a chemically defined diet intragastrically (IG), an equivalent diet intravenously (IV) and solid food orally (CH) for 8 days, and their small intestines and colons compared. All received equal calories. The small intestine was divided into equal proximal (A), middle (B), and distal (C) segments for measurements. Mucosal weight per cm in segments A and B of IG were, respectively, 65 and 38% higher than in IV (P less than 0.01), but 27 and 33% lower than in CH (P less than 0.01). However, in the distal segment, C, mucosal weight in IG was similar to IV and CH was 79% higher (P less than 0.01). DNA and protein followed the same pattern. Segment A sucrase activities were similar in CH and IG and were much higher than in IV (P less than 0.01). Sucrase in IG dropped very rapidly distally so that it became much lower than in CH (P less than 0.05) and similar to IV. Mucosal weight, DNA, and protein in the colon were not significantly different in IG and IV, which were both significantly lower than in CH (P less than 0.01). The results indicate that a chemically defined diet maintains intestinal mass well in the proximal small intestine, but the effect diminishes rapidly in a distal direction so that distal small intestine and colon become atrophied and similar to those in intravenous feeding.