The mucosal distribution of G cells was quantitatively mapped in resected stomachs from 42 patients (12 with gastric ulcer, 11 with duodenal ulcer, 14 with duodenal ulcer and uremia, and 5 with gastric cancer). Along the histological border of the proximal part of the pyloric antrum there was in all patient categories a transitional zone of varying extent, with a low G-cell density before the cells disappeared in the body of the stomach. The proximal end of the duodenum contained considerably fewer G cells than in the antrum, and the number was virtually equal in all groups. Within the antrum there was in the material as a whole a gradual increase in G-cell density from the proximal to the distal part, but this difference was not apparent for the gastric ulcer patients. When corresponding antral segments were compared between the various patient groups, the G-cell density was found to be significantly decreased in the distal antrum of the gastric ulcer patients. In all patient categories, except the duodenal ulcer group with uremia, the circumferential distribution of G cells showed reduced density along the curvatura minor. For the material as a whole there were great individual variations in the overall antral G-cell density, in the antral area corresponding to the distribution of G cells and in the total G-cell mass; these three variables were not significantly related to diagnosis, age or sex.