In two patients with malignant gastrinoma and the Zollinger-Ellison syndrome, we were able to use selective arterial stimulation with secretin as a technique to localize the lesions accurately, allowing resection. The technique of selected arterial secretin stimulation is one of measuring variations in gastrin levels in both the hepatic vein and a peripheral artery at specified times after injection of secretin into a specific artery. When the criteria for localization have been met, one can plot the presence of the gastrinoma within the blood supply of the injected artery and, using angiograms, thus accurately localize the lesion. This method promises to be a valuable additional tumor-localizing procedure, particularly when gastrinomas are extrapancreatic.