The biochemical characteristics of a pancreatic tumor from a patient with watery diarrhea, hypokalemia, hypochlorhydria, and steatorrhea is described Plasma concentrations of vasoactive intestinal peptide (VIP) were elevated 6-fold, those of neurotensin (NT) were elevated 10-fold, and those of pancreatic polypeptide (PP) were elevated 200-fold above the normal range. The pancreatic tumor removed was found to contain high concentrations of these peptides in a similar ratio to plasma. The tumor content of NT was 6 times higher than any previously reported, but no specific symptoms could be ascribed to NT. After removal of the tumor, plasma levels of VIP, NT, and PP returned to normal, and the diarrhea disappeared. Gel chromatography of plasma and tumor extracts indicated that all of the immunoreactive PP co-eluted with standard human PP. When the tumor extract was subjected to gel chromatography and high pressure liquid chromatography, two forms of VIP were detected, the major one resembling porcine VIP and a smaller more hydrophobic form detected by C- but not N-terminally directed VIP antisera. This smaller form was not present in normal ileum. Immunoreactive NT in plasma was predominantly an N-terminal fragment. High pressure liquid chromatography of the tumor extract revealed that approximately 75% of the NT immunoreactivity consisted of N-terminal fragments. In contrast, normal ileum contained only authentic NT-(1-13). Since N-terminal fragments of NT are thought to be biologically inactive, the nature of the immunoreactive NT should be determined before attempting to assign specific clinical symptoms to NT-secreting tumors.