Various mucosal lesion induced in the glandular stomach of female Wistar rat by oral N-methyl-N'-nitrosoguanidine (MNNG) were examined by autoradiography, and the process of gastric carcinogenesis was studied. Hyperplastic polyp, which often occurred in the distal pyloric glandular region, was highly complicated by adenoma and cancer. Autoradiography revealed several labelled cellular masses near the surface of the polyp, suggesting the involvement of these masses in carcinogenesis. Deep erosions were noted in the proximal pyloric region, but no carcinogenesis was noted except for regenerative atypical tissue and an accumulation of labeled cells. Downward proliferation (slipping-in), which occurred in the proximal pyloric glandular region, showed a high labelability in autoradiography. In this experiment, only downward adenomatous hyperplasia was formed in the submucosal layer, but carcinogenesis was considered possible in the stomach too, as submucosal cancer was noted in the small intestine. Since these mucosal lesions in rats can also be observed in the process of human gastric carcinogenesis, carcinogenetic experiments with MNNG in rats are considered useful for elucidating the process of human gastric carcinogenesis.