The purpose of this project is to define hormonal factors responsible for gastric hypersecretion of acid in duodenal ulcer disease in man. The hypothesis to be tested is that circulating concentrations of one or more of the molecular forms of gastrin or another hormone distinct from gastrin which stimulates gastric acid secretion are abnormally increased due to overproduction, abnormal autoregulation, or decreased rates of metabolism. Studies to be performed include bioassay of plasma extracts for acid stimulants distinct from gastrin; study of biological activity, metabolism and release of molecular forms of gastrin; use of specific amino acids as gastrin-releasing agents; analysis of provocative tests for diagnosis of gastrin-secreting tumors; and in vitro studies of gastrin synthesis and binding to tissue receptors.