In a series of experiments in rats, the movement of the advancing sheet of epithelial cells streaming from a border region into the woind bed of standardized defects in the glandular mucosa of the stomach was studied using the light microscope and the scanning electron microscope (SEM). A very high synthetic activity of mucus in gastric lining cells of the border region (with a typical shift in its reaction product towards a more acidly reacting product showing metachromasia with toluidine blue) was observed histochemically, together with a loss of characteristic features of glandular cells in the border region with an increase in mitotic activity. The SEM images enabled a detailed analysis of the advancing epithelial front in its relation to the border regions. Considerable local differences were noted in the moving pattern of the migrating epithelium, possibly in relation with the nature of the base on which it has to move. The lesion of 3 mm diameter appeared to close usually after 10 days; in some instances, however, small epithelial defects were later found which could easily be traced with the SEM.