The effects of unavailable carbohydrate on the cecum and colon were investigated in rats raised on diets containing 20% of glucomannan or cellulose for 8 weeks. Glucomannan ingestion caused the increase of cecum and colon weights, and the effect was greater in the cecum than in the colon. However, on ingestion of cellulose, a remarkable increase in tissue weight was observed in the colon rather than in the cecum. Measurement of DNA, RNA and protein in cecal and colonic mucosa demonstrated that the cecal enlargement caused by glucomannan was dependent on the increases in both number and size of mucosal cells, and that the cecal and colonic enlargements caused by cellulose and the colonic enlargement caused by glucomannan resulted from an increase in the number of mucosal cells; i.e., hyperplasia. (Na + K)ATPase activity of the cecal and colonic mucosa in the cellulose and glucomannan groups was significantly higher than that in the control group. Thus, (Na + K)ATPase activity was increased proportionally to the intensity of the process of tissue enlargement.