The mucosal potential difference in the body of the stomach was measured in 18 patients with gastric ulcers, five with gastritis, and three with pernicious anaemia. The results were compared with those of 12 normal volunteers. Significantly lower than normal levels were found in the groups of patients with gastric ulcers and gastritis. The patients with pernicious anaemia had even lower potential differences. In an additional 25 subjects, measurements were made in the duodenal bulb and at five sites in the stomach under direct vision at endoscopy. Biopsies were taken from each site for histology. Subjects with gastritis at the high lesser curve had a significantly lower potential difference at that site than those without gastritis. In the latter group, the potential difference at the high lesser curve was as low as in the antrum, and invariably lower than at the high greater curve.