Gastric secretory response to betazole hydrochloride, administered intramuscularly, and to an intravenous calcium gluconate infusion was studied in four patients with histologically proven nonbeta cell pancreatic tumors. Five duodenal ulcer patients with comparable levels of basal secretion were also studied. Calcium was a potent stimulus for gastric secretion in patients with gastrin secreting tumors, as the acid response to calcium infusion equalled or exceeded the maximal acid response to betazole. In contrast, the acid response to calcium infusion in the duodenal ulcer patients was a third of the response to betazole. Calcium acts to potentiate the response to gastrin and stimulates the release of gastrin from non-beta cell tumors. The acid response to calcium infusion may be a useful screening test for the Zollinger-Ellison syndrome.