Thirty-four rats were sacrificed periodically from 191th day to 378th day after peroral administration of MNNG. 94 lesions were found in pyloric region of the glandular stomach of rats. Histologically, these lesions were classiffied as follows: 1) Regenerative glandular hyperplasia (34 lesions), 2) Adenomatous glandular hyperplasia (44 lesions), 3) Adenocarcinoma (16 lesions). The changes of cellular proliferation and differentiation of these lesions were studied by means of the (3)H-thymidine radioautography. 1. At the margin of these three types of lesions, the tendency of increase of labelled cells and extension of generative cell zone toward mucosal surface was observed. 2. The regularities of cell proliferation and migration from neck cell to foveolar and surface cell in normal gastric glands were maintained in the regenerative glandular hyperplasia and in the adenomatous glandular hyperplasia but in the adenocarcinoma such normal cell kinetics were lacking. 3. In these three types of lesions, two kinds of different atypical cells were found, namely dark and red cell, which can not be found in normal gastric gland. They were distinguished not only morphologically but also according to the degree of (3)H-thymidine uptake. The dark cell has close similarities to the neck cell and red cell to the regenerative or foveolar epitheliar cell. MNNG induced adenocarcinomas were composed of these dark and red cells which had marked cellular and structural atypia. It is suggested that atypical proliferation of the neck cell which is induced by MNNG during repeated injuries and regeneration of gastric mucosa, might develop into the dark and red cell and finally into adenocarcinoma.