In preliminary experiments male rats fed 10 to 20 per cent cellulose diets were found to outlive the same sex fed a stock diet. This increase in the life span of these animals was probably due to the slow rate at which they attained maturity rather than to a direct influence of the cellulose. However, it shows that the rat intestine is capable of tolerating these extreme levels of cellulose for a period in excess of that regarded as the normal life span. After a series of trials it proved possible to adjust the composition of the diet so that as good growth was obtained with diets containing 20 per cent cellulose or agar as with diets lacking roughage. Mice can also tolerate such levels of roughage and grow normally. In the course of an epidemic of diarrhea among the mice, it was observed that the animals on the high cellulose diet were not affected. In some rats signs of intestinal irritation were found. These seem to be characteristic of individuals. The more finely ground cellulose produced the least irritation. Rice hulls were no more irritating than bran or regenerated cellulose. No evidence was found that this irritation leads to permanent injury. In studies to determine the relative digestibility of crude fiber from beet pulp, bran and regenerated cellulose, the beet pulp was found to be digested to the extent of 40 per cent, while the other two were about half as well digested. In the case of bran and regeneratd cellulose 3 per cent levels are slightly better digested than 6 per cent ones. In a determination of the effect of the cellulose ingested upon the feces excreted it was found that the dry weight of these feces increases beyond an amount that can be ascribed to the ingested cellulose. Either there is a decreased digestibility of nutrients or the cellulose causes increased losses in the feces of such materials as epithelial cells from the intestine. No relation could be established between the moisture content of the feces and the level of roughage ingested, except in the case of agar. In the case of this material there was a direct relationship.