Sodium polyacrylate (PANa) is a water-soluble, high-molecular compound, and its aqueous solution shows a very high viscosity and stringiness. In the present study, preventive effects of PANa on three kinds of esophageal lesions induced by gastric juice were examined in comparison with those of aceglutamide aluminium and sodium alginate. The influences of PANa on gastric contents were also studied. The preventive effect of PANa given intraesophageally on esophageal lesions induced by the intraesophageal application of gastric juice was more potent than aceglutamide aluminium and sodium alginate. Oral administration of PANa inhibited the formation of esophageal ulcer by pylorus ligation more markedly than aceglutamide aluminium, whereas sodium alginate had no effect in a high dose of 500 mg/kg. In preventing gastric ulcer which occurred simultaneously with the esophageal ulcer after the pylorus ligation, aceglutamide aluminium was most potent, and PANa was as potent as sodium alginate. Oral administration of PANa showed a more protective effect than aceglutamide aluminium on the esophageal ulceration induced by the simultaneous ligations of the pylorus and limiting ridge, whereas sodium alginate in a high dose of 500 mg/kg had little effect on the ulcer formation. PANa caused only a slight increase in the pH of gastric juice and a slight decrease in pepsin activity. From these results, it may be concluded that PANa showed an antiulcerogenic activity mainly due to its mucosa covering action against gastric juice.