Deep mucosal wounds were produced within 1 mm of the squamo-columnar junction of the stomach of the rat, either on the squamous or on the glandular side, and the response of both the regenerating epithelium and of the adjoining, dissimilar epithelium was studied 24 hours to 12 weeks later. Squamous epithelium, both overlying previously injured squamous areas and adjoining previously injured glandular areas showed distinct hyperplasia that began on the 3rd day and was still present at 12 weeks. Glandular epithelium overlying previously injured glandular areas and adjoining previously injured squamous areas showed no hyperplasia but modulation of parietal and chief cells into mucus-secreting cells with increased mitosis. These changes appear more likely the effects of non-specific mitogenic agents than of reduced concentration of epithelial chalones.