Histamine H2 receptor antagonists have been reported to protect the gastric mucosa of animals and humans against aspirin-induced damage. It is unclear, however, whether this protective effect can be observed at doses less than those needed to markedly inhibit gastric acid secretion. We have developed a single-dose endoscopic assay system of aspirin-induced gastric mucosal injury in normal conscious dogs. In this model, severe gastric mucosal injury and a decrease in the pH of the gastric luminal contents were consistently demonstrated 2 h after the oral administration of 100 mg/kg of aspirin. Pretreatment with three histamine H2 receptor antagonists (cimetidine, ranitidine, BMY-25271), prevented both of these effects in a dose-related manner. All three H2 receptor antagonists reduced gastric mucosal injury only at doses that were greater than those required to prevent the aspirin-induced decrease in gastric luminal pH or to inhibit histamine-stimulated gastric acid secretion in Heidenhain pouch dogs. Plasma levels of aspirin were not altered by histamine H2 receptor antagonism. These results indicate that acid inhibition is an important component of the mechanisms whereby histamine H2 receptor antagonists protect the gastric mucosa from aspirin-induced damage in the dog.