Localization and peptide content of endocrine pancreatic tumors. Endocrine pancreatic tumors contain and frequently secret neurohormonal peptides. This phenomenon can be used as a diagnostic and classifying tool. This study analyzes 31 patients operated on because of an endocrine pancreatic tumor, including the diagnostic procedures and the localization methods. In 15 insulinoma cases only 6 patients had a positive arteriography, while all 11 selective pancreatic vein samplings were positive. The immunoreactivity showed that, besides insulin, most tumors also contained other peptides. Of four gastrinoma cases the arteriography was positive in three, but the selective vein sampling localized the tumor in all. The tumor's content of peptides showed mixed patterns. In the four glucagonomas, the arteriography was positive in all and the venous sampling performed in three of the cases also was positive. In five pancreatic polypeptide-containing tumors (PP-omas) the arteriography was positive in four and sampling performed in two was positive in both. In the PP-omas the peptide pattern showed that these tumors frequently contain several peptides. We used selective pancreatic vein sampling in 21 cases with positive result in all. In the cases in which arteriography was negative, the sampling results helped the surgeon to find the tumor. The peptide pattern in the tumors varied greatly and most tumors were multihormonal. 