In 10 normal subjects, in 32 patients with duodenal ulcer (DU), and in 11 patients with partial gastrectomy (Billroth I), serum gastrin rose significantly after an oral and intraduodenal test meal. The highest increases were observed in DU patients after the oral as well as after the intraduodenal test meal. After the intraduodenal test meal in 4 normal subjects and in 17 DU patients an increase of gastric acid secretion and serum gastrin was measured. In basal state, after an intraduodenal or an oral test meal, DU patients with normal gastric acid secretory capacity had higher serum gastrin concentrations than DU patients with gastric hypersecretion. There was a good correlation between peak serum gastrin levels after the oral and after the intraduodenal test meal. From these data it is concluded: (1) Intraduodenal application of a test meal results in release of gastrin from extragastric sites. (2) Extragastric gastrin is biologically active. (3) DU patients are able to release more antral and more extragastric gastrin in response to a test meal. Further studies, however, are necessary to show the significance of these findings in the pathogenesis of peptic ulcer disease.