In an attempt to study variations in the size of epithelial cell populations in the intestinal mucosa, these cells were isolated from the mucosa of small intestine of male mice (Db strain) weighing around 20g, by the use of Ranvier's 1/3 ethylalcohol, and the total number of epithelial cells was estimated under various experimental conditions, after counting these cells by the hemocytometer. The chief results obtained are as follows: 1. In the mice fasted for 5 days, the total number of epithelial cells isolated from the mucosa of small intestine was gradually decreased, reaching to a minimum value of about 1/6 of the normal levels at 5 days. 2. Following total-body x-irradiation in a dose of 600r, the total number of eithelial cells in the small intestine was reduced abruptly and a minimum volue of about 1/8 of the normal levels was reached within several days. 3. In mice that received a single intraperitoneal injection of either Mitomycin-C, Endoxan, or Thio-TEPA, in a dose of 5 mg/kg, 160 mg/kg, 15 mg/kg, respectively, the size of epithelial cell populations in the small intestine was rapidly diminished in a manner similar to that observed after total-body X-irradiaton. However, the rate of reduction in the epithelial cells populations was greater in these cases than in the case of X-irradiation, except for in the case of treatment with Endoxan. 4. The observed reduction in the size of epithelial cell populations of the small intestine is due chiefly to inhibition of mitosis of epithelial cells in the crypts and partly to destruction of proliferating epithelial cells in these areas.