Electron microscopic study was made on precancerous and cancerous lesions in glandular stomach of rats induced by MNNG. Three types of lesions, were found; regenerative glandular hyperplasia, adenomatous hyperplasia and adenocarcinoma. These were compared with the fine structure of similar lesions produced by X‐irradiation in the glandular stomach of mice. The precancerous lesion in the present study demonstrated formation of adenomatous hyperplasia by epithelial cells which included mucoid cells characterized by sparce surface microvilli, a few erratic terminal webs, small number of mucous granules, many small mitochondria, and a bizarre nucleus. The development of these mucoid cells seems to be related with the growth of gastric carcinoma. The ultrastructure of adenocarcinomas revealed many structural variations or abnormalities of cellular differentiation. They were classified into 1) surface and pit mucous cell type, 2) pyloric gland cell type, 3) goblet cell type, 4) paneth cell type, 5) intestinal epithelial cell type, 6) endocrine cell type, 7) oncocyte type, 8) filament‐rich cell type, and 9) anaplastic cell type involving intracellular microcyst cells. Filament‐rich cells were found in poorly differentiated adenocarcinoma or scirrhous carcinoma and may be derived from metaplasia of tumor