Somatostatin in gastric juice was determined in normal subjects and patients with duodenal ulcer. Gel exclusion chromatography of gastric juice revealed that the main immunoreactivity existed at the position of somatostatin-14. A large amount of somatostatin was present in gastric juice, and the quantity increased following tetragastrin stimulation. Furthermore, there was a good inverse correlation between somatostatin concentration and acidity of gastric juice; however, there was no difference between normal subjects and patients with duodenal ulcer in the amount of somatostatin released into gastric juice.