The total concentration of gastrin and distribution of gastrin components were examined in mucosal biopsies from corpus, antrum, duodenum, and jejunum from normal subjects and patients with duodenal ulcer. The concentration was highest in the antrum, being 12.1+/-1.9 nmol per g of mucosa (mean +/-SEM) for normal subjects, and 9.0 +/-1.6 nmol per g of mucosa for duodenal ulcer patients (P eaual to 0.03). A steep gradient was found distally: in the proximal duodenum the concentration was 0.1; in the distal duodenum, 0.02 to 0.01; and in the proximal jejunum, less than 0.01 of the antral concentration. In corpus of the stomach, the concentrations were similar to those found in the jejunum. Gel filtrations showed that most gastrin immunoreactivity was eluted in positions corresponding to serum component II (gastrin-34-like) and III (gastrin-17-like), but immunoreactivity corresponding to all the components present in serum was found. No interference from cholecystokinin was observed in duodenal biopsies. In corpus, antrum, and jejunum component III was the predominant form, whereas component II made up half of immunoreactive gastrin in the duodenum. No major differences were observed between normal subjects and duodenal ulcer patients. There was no simple relationship between acid secretion and mucosal gastrin concentration, but ulcer patients with the highest acid secretion had the lowest antral content and the highest duodenal content.