Gastritis is associated with peptic ulcer, gastroenterostomy, pernicious anemia, and exposure to nitrosamines. Once established, the process may be self-perpetuating, resulting in atrophy, metaplasia, dysplasia, and neoplasia. This can be explained by the process of endogenous nitrosation of amines in the inflamed gastric mucosa. Evidence is presented to support this hypothesis. Several drugs given parenterally have been identified as mutagenic nitroso compounds in homogenates of human and canine antral mucosa. Nitrite for this process is apparently derived from the inflamed mucosa. Different amines appear to be nitrosated at different places in the antrum, suggesting the presence of site-specific enzymes that control these reactions.