To clarify the organo-specificity of small intestinal cancer, Wistar strain male rats were operated on as follows: Interposition of ileal loop in distal colon for group I and simple laparotomy for group II. MNNG was given via the rectum at the dose of 2.5 mg/day for 2 weeks from the second postoperative week. After intravenous injection of BrdU 50 mg/kg 40 weeks thereafter, these rats were sacrificed. Tumor incidence was almost the same (77% and 78% for group I and II, respectively), but with an obvious difference in site of occurrence, and the number of tumors per unit area was 0.153 in large intestine and 0.015 in interposed ileum for group I and 0.119 in large intestine for group II. Labeling indices of BrdU in the interposed ileal mucosa and control ileal mucosa were 8.2 and 9.7%, respectively without a great difference there between, but were significantly higher compared with 5.1% in colonic mucosa. The above results suggested possible resistance of the interposed ileal mucosa to the provocation of tumor by MNNG compared with the colonic mucosa because of quick cell turnover of the epithelium in the small intestinal mucosa.