The gross and histopathologic findings seen in the stomach of rats treated with an H2-antagonist, tiotidine, will be presented. The data presented is from two studies, a six month oral (gavage) and dietary study in male rats, and a 12 month oral chronic toxicity study combined with a 24 month oncogenicity feeding study in rats. Significant findings in the stomach included fundic glandular dilatation, parietal cell atrophy, metaplastic fundic cells, fundic nuclear clusters, and dysplasia/carcinoma lesions. The chronology of the borderline type dysplasia/carcinoma lesions starting from the very early to the more advanced infiltrating carcinomas will be shown. A total of 17 rats with lesions of the dysplasia/carcinoma complex out of 828 treated rats from the two studies were present. No similar type lesions were seen in any of the 432 control rats examined. The importance of the diagnostic methods used in the examination of the stomach is stressed.