The acid-secreting gastric mucosa resists intraluminal acid better than the nonsecreting. Here we investigated pH at the epithelial cell surface, mucosal permeability, and blood flow during intraluminal administration of acid (100 mM) in acid-stimulated and nonstimulated gastric corpus mucosae. Surface pH (H⁺-selective microelectrodes), permeability (clearance of ⁵¹Cr-EDTA), and mucosal blood flow (laser-Doppler flowmetry) were studied in Inactin-anesthetized rats. Acid secretion was stimulated with pentagastrin (40 μg · kg⁻¹ · h⁻¹) or impromidine (500 μg · kg⁻¹ · h⁻¹), or HCO₃ ⁻ (5 mmol · kg⁻¹ · h⁻¹) given intravenously. Surface pH was only slightly reduced by intraluminal acid in acid secretion-stimulated or HCO₃ ⁻-treated rats but was substantially lowered in nonstimulated rats. Clearance increased threefold and blood flow increased by ≈75% in nonstimulated rats. During stimulated acid secretion or intravenous infusion of HCO₃ ⁻, clearance was unchanged and blood flow increased by only ≈30% during intraluminal acid. Increased epithelial transport of HCO₃ ⁻ buffering the mucus gel is most probably the explanation for the acid-secreting mucosa being less vulnerable to intraluminal acid than the nonsecreting.