This protocol describes the use of histamine H2-receptor antagonists to control gastric acid hypersecretion in patients with Zollinger-Ellison syndrome. It details which patients will be considered for treatment with these agents, the pretreatment procedures and the procedures to be followed in establishing the proper intravenous dose of histamine H2-receptor antagonist. It also details the procedure to be used to establish a safe and effective oral long-term maintenance dose of either cimetidine, ranitidine, nizatidine, or famotidine. By following these procedures it will be possible to evaluate the effectiveness of intravenous histamine H2 therapy if it is determined this is important for antisecretory control during periods when patients cannot take oral gastric antisecretory agents. It will also be possible to evaluate the effectiveness of histamine H2-receptor in patients with Zollinger-Ellison syndrome after successful gastrinoma resection who continue to have mild gastric hypersecretion.