Luminal and mucosal pH were measured endoscopically in patients with reflux esophagitis and antral gastritis and in control subjects. In all subjects, significant lumen-to-mucosa gradients were observed in the esophagus, stomach and acidified proximal duodenum. In the reflux patients luminal pH was lower in the fundus (mean +/- SEM, control vs. reflux esophagitis: 2.01 +/- 0.17 vs. 1.32 +/- 0.18; p less than 0.02) and antrum (3.51 +/- 0.35 vs. 2.13 +/- 0.24; p less than 0.01) and, in the gastritis patients, in the fundus (2.01 +/- 0.17 vs. 1.3 +/- 0.17; p less than 0.02). In both patient groups, mucosal pH was lower in the fundus (control vs. reflux vs. gastritis: 4.84 +/- 0.37 vs. 3.37 +/- 0.61 vs. 3.12 +/- 0.6; p less than 0.05) and acidified duodenal cap (6.74 +/- 0.13 vs. 6.09 +/- 0.24 vs. 5.73 +/- 0.46; p less than 0.03). Mucosal pH profiles at the various sites showed less resistance of the gradient to a highly acidic environment in both the lower esophagus and antrum than in fundus and duodenum, and this was the case in the patient and control groups. Though associated with a more acid environment, neither esophagitis nor antral gastritis exhibits a specific deficit in the 'mucus-bicarbonate barrier', suggesting that the pathogenesis of these disorders may depend more on abnormal 'attack' rather than impaired defense.