In an attempt to elucidate histogenesis of stomach cancer, quantitative analysis and measurement of DNA contents of various atypical lesions were sequentially made in the process of gastric carcinogenesis of Wistar strain of rats. Along with this, the effect of vagotomy on the development of atypical or neoplastic lesions were studied. A variety of focal lesions in the glandular stomach were seen in the middle or 4 and 12 weeks after the oral administration of N-methyl-N'-nitrosoguanidine (MNNG, 83 mg/l in drinking water) for 25 weeks. Both upward and downward growth was found in the intramucosal atypical lesions as well as frank carcinoma; the former lesions were histologically classified into 3 (Type I--Type III). On the basis of DNA distribution pattern, Type III lesions were considered to be intramucosal carcinoma and Type II to include precancerous state in some instances. In a group of rats vagotomized 1 week prior to the start of MNNG administration, there were significantly more lesions than in a group of MNNG alone. In contrast to the latter group which developed lesions in an uniform distribution pattern along the lesser curvature in the pyloric region, lesions in the former were characterized by random distribution pattern.