In the previous paper, the content of prostaglandin D2 (PGD2) was found to be five times higher than that of PGF2α or PGE2 in rat gastric mucosa. Thus, the effect of exogenous PGD2 on ethanol-induced gastric mucosal lesions was studied. Intragastric administration of 500μg/kg of PGD2, which did not inhibit gastric acid secretion, but did stimulate pepsin secretion, significantly prevented ethanol-induced ulcer formation compared with the control group. Although 100μg/kg of PGD2, also given intragastrically, prevented ethanol-induced ulcer formation compared with the control group, the difference was not statistically significant. Gastric mucosal blood flow (GMBF) in the corpus ventriculi measured by the hydrogen gas clearance technique was significantly increased when PGD2 was given intragastrically at 100 or 500μg/kg. These results suggest that PGD2 has anti-ulcerogenic effect in the stomach and that an increase in GMBF might be one of factors by which this effect is mediated.