Rats were given MNNG perorally and were killed periodically. In the effective 69 rats, after 26 weeks, 67 lesions of glandular epithelial proliferation developed in glandular stomach of rats. These lesions were classified in three types, as follow: 1) Type A (43 lesions) with scanty cellular and structural atypism. 2) Type B (20 lesions) with moderate grade of atypism. 3) Type C (4 lesions) with severe atypism which can be easily diagnosed as carcinomatous. In histological and electron microscopical aspect of their stroma, type A lesions are surrounded by prominent formation of both fibroblast sheath and basement membrane. However, type B lesions exihibit the reminiscent or absence of basement membrane, as well as fibroblast sheath, especially in the following situation: 1) cytoplasmic protrusion into the extra-epithelial space. 2) occurrence of polymorphonuclear leucocytes adjacent of the epithelium 3) fibroblast directly contact with the epithelium 4) basement membrane appeared corrugated and multiple layered, which is similar to the human gastric cancer, mainly as poorly differentiated adenocarcinoma. In the present time, histological criteria of the experimental stomach cancer is not clear. But, in this study the author suggest that the type B lesions can be concluded to stomach carcinoma, because various changed of the basement membranes in the type B lesions are similar to poorly differentiated adenocarcinoma of human gastric cancer. The changing mode of the basement membrane is contributed not only by epithelical cells but also by mesenchymal cells, especially fibroblasts. Moreover MNNG could affect both these two types of cells, because various mesenchymal tumors are produced.