The total number of gastric and duodenal gastrin cells was determined in the gastrectomy specimens from 8 patients with peptic ulcer. Planimetry was used to determine the antral and duodenal surface. The immunoperoxidase method with specific antigastrin antibodies was used for staining gastrin cells, and the mean concentration of nucleated gastrin cells/mm2 of antral and duodenal surface was determined by light microscopy. The mean number of duodenal gastrin cells in the resected duodenum was 4.8% of the total gastric gastrin cell mass. The concentration of gastrin cells in the antrum was quite variable from 1 mucosal site to another. The degree of extension of the antral gastritic areas was a major factor influencing the mean concentration value. The total number of gastrin cells in the stomach varied from approximately 7-74 million cells. Hyperplasia of gastrin cells cannot be demonstrated by studies performed in small specimens taken for biopsy.