The pharmacological actions of sodium alginate on the mucosa of the upper gastrointestinal tract were studied by in vivo and in vitro experiments. To induce the experimental gastric erosion, rats were administered orally with 500 mg/kg of aspirin, and subsequently treated with oral administration of 5% (wt/vol) solution of sodium alginate. Histopathological observation of the stomach of these rats showed that sodium alginate formed a thick layer and covered the foci of aspirin-induced erosion. The layer of sodium alginate may restrain the destruction of tissues and hemorrhage at the foci of erosion. The digestion of the mucosa of the excised rat stomach or rabbit esophagus in artificial gastric juice was highly inhibited when the mucosa was previously treated with 5% solution of sodium alginate. This was evidenced by the fact that significantly fewer amounts of tyrosine was separated into the incubation medium from the alginate-treated stomach or esophagus as compared with untreated controls. Sodium alginate showed only weak antacid or no anti-pepsin actions. The layer of sodium alginate covering the lesion may inhibit the lytic action of pepsin and HCl and may protect the mucosal surface of the stomach and esophagus mainly by its mechanical action.