Serial autoradiographic studies were made in wistar-rats during oral administration of N-methyl-N'-nitroso-guanidine (MNNG). Between the 4th and the 40th week 16 rats were killed at intervals. During the first period of the experiment we observed a foveolar hyperplasia of the mucosa of the glandular stomach. Later we found adenomatous hyperplasia but finally severe mucosal dysplasiae, which were looked upon as precursors of cancer. In the course of the 40th weeks investigation period no cancer was induced. In the antral mucosa the labelling index was higher than in the other parts of the glandular stomach. On the lesser curvature the proliferation rate was higher than on the greater curvature, or in comparison with the anterior and the posterior wall of the stomach. Furthermore the severe mucosal dysplasiae showed a higher labelling index than the adjacent area or the corresponding part of the lesser curvature.