Gastrinomas were localized by concurrent blood sampling in the hepatic vein and portal vein tributaries in 10 of 12 Zollinger-Ellison patients. In one patient, the portal vein could not be catheterized; in the other, a gastrin gradient could not be picked up in either of two examinations. Six of the patients were subsequently operated upon: 5 had pancreatoduodenal resection and 1 had laparotomy at which metastases were found. Four of the resections were probably curative as the patients have done well without treatment since with concentrations of gastrin in serum near zero. The observation period ranges from 17 to 20 mo. It is concluded that transhepatic catheterization of the hepatic vein and portal vein tributaries with blood sampling for gastrin determination permit the verification of the Zollinger-Ellison diagnosis and the localization of the gastrinomas. The latter may lead to the selection of a curative operative treatment in some of the patients.