The effects of aspirin and treatment with geranylgeranylacetone (GGA), an antiulcer drug, on the content of gastric glycoproteins were investigated in pylorus-ligated rats. In normal rats, the amount of gastric glycoproteins in the mucous layer was .apprx. 1.5 times higher than that in the gastric mucosa, indicating that the glycoproteins were distributed in the mucous layer as a highly concentrated state. Aspirin (100 mg/kg, orally) decreased the content of gastric glycoproteins both in the mucous layer and in the gastric mucosa. The amount of the macromolecular fraction in the gastric juice, which correspondedd to the gastric glycoproteins on the basis of molecular size, was not affected by aspirin. GGA (300 mg/kg, [intraduodenally]) could prevent the decreases of the total amount of gastric glycoproteins in the mucous layer plus gastric mucosa. The glycoproteins coating the surface of the gastric mucosa may play a role as a defensive mechanism. GGA exerted an antiulcer effect on aspirin-induced mucosal damage through preventing the decreases in gastric glycoproteins.