The mucus-bicarbonate barrier provides the first line of defense against potentially harmful gastric luminal contents. Its integrity can be assessed in man by demonstrating the presence of a pH gradient across the mucus gel layer, from the acidified lumen to near-neutral pH at the mucosa. Our aim was, firstly, to assess the effects of ethyl alcohol and bismuth subsalicylate on the integrity of this lumen-to-mucosal pH gradient and, secondly, to evaluate whether pretreatment with bismuth subsalicylate would protect against any deleterious effects of ethyl alcohol. Ten healthy adults underwent two upper endoscopic procedures with microelectrode measurement of juxtamucosal pH gradients. At the first endoscopy, the effects of 30 ml of bismuth subsalicylate on the gradient was evaluated. At the second endoscopy, gradients were measured before and after luminal installation of 60 ml of ethyl alcohol (40% v/v) and following pretreatment with either 30 ml of bismuth subsalicylate or placebo. pH measurements were technically easy to perform and provided consistently reproducible results. A distinct juxtamucosal pH gradient (pH 4.0 +/- 0.2 units) was identified in all subjects in the basal state. Neither bismuth subsalicylate nor ethyl alcohol had a significant effect on these gradients. We conclude that a distinct pH gradient between gastric luminal fluid and the mucus gel layer can be readily demonstrated in man. Neither bismuth subsalicylate nor ethyl alcohol have a significant effect on this gradient.