The gastrin hypothesis. Implications for antisecretory drug selection. Newer potent and long-acting inhibitors of acid secretion, such as the proton pump inhibitor omeprazole, are becoming available for general use. These drugs promise to control acid-peptic disease effectively in patients who do not respond adequately to conventional short-acting H2-receptor antagonists. The safety of chronic administration of these drugs has come into question, however. Lifelong profound inhibition of acid secretion in rats induced by superpotent inhibitors of acid secretion or subtotal fundectomy is associated with the development of carcinoid tumors of enterochromaffin-like (ECL) cells in the gastric corpus. Available evidence supports a role of gastrin, which becomes chronically elevated in animals subjected to prolonged and profound hypochlorhydria. In humans, hypergastrinemic states such as Zollinger-Ellison syndrome and atrophic gastritis are associated with an increased risk of ECL-cell carcinoid tumors. Such observations have raised concern that humans may also be susceptible to carcinoid tumor formation in response to potent inhibitors of acid secretion. To date, however, no cases of carcinoid tumor have been attributed to the use of omeprazole in humans. If achlorhydric doses are not used, significant hypergastrinemia can be avoided while effectiveness of treatment is maintained. Such measures should minimize any risk of ECL-cell carcinoid tumors in humans taking potent long-term antisecretory drugs. 