The fine structure of gastric epithelial cells of normal rats and of rats restrained at low temperature has been examined by transmission and scanning electron microscopy. The observation of the normal mucosa has revealed that in comparison with the cells overlying the flat surface of the stomach, those covering the plicae have a different surface structure with numerous microvilli and a peculiar organization of intercellular junctions. Restrained animals initially showed swelling of the foveolae that was followed by a detachment of the superficial cells leading to severe stomach erosions. It is postulated that the first modification induced by stress is a decrease in the amount of mucus granules and a weakness of the mucous protective barrier. This was confirmed by the results obtained by pretreating rats with zolimidine. This substance, which has been shown to increase secretion and synthesis of gastric mucus, completely protected the animals from the stress-induced damage of the stomach surface. Furthermore, this treatment produced an enlargement of the cisternae of the endoplasmic reticulum and an increase in the surface of the Golgi apparatus in the epithelial cells.