The effect of luminal application of acetylsalicylic acid (ASA) on the liberation of gastric mucosal barrier constituents at pH 5.0 and 2.0 was investigated. The Lucite chamber stomach-flap preparation was used in 18 dogs whose basal H+ secretion was inhibited by cimetidine. An ASA dose of 10 mmol at pH 5.0 caused a moderate increase in content of proteins, glycoproteins, and glycolipids in the instillates. This was accompanied by an increase in transmucosal potential difference (PD) without concomitant changes in the appearance of the gastric mucosa. However, the content of mannose used as an indicator of plasma leakage remained unchanged. A 20 mmol ASA dose at pH 5.0 produced further enrichment of the instillates in mucus constituents, and an increase in mannose content was observed, but PD still remained elevated. In contrast, the same dose of ASA at pH 2.0 severely depleted the gastric mucosal barrier of its mucus constituents and caused a marked increase in leakage of plasma elements. These changes were accompanied by a substantial drop of PD and gastric mucosal damage. These data indicate that the topical application of ASA causes the liberation of mucus constituents, which results in weakening of the gastric mucosal barrier and thus facilitates the pepsin and acid penetration of gastric mucosa. The extent of mucosal damage caused by aspirin strongly depends on the pH of luminal exposure.