The availability of pure intestinal hormones and the development of radioimmunoassays for their measurement has expedited research into many aspects of gastrointestinal endocrinology. A complex balance evidently exists between the different intestinal hormones and also the rest of the endocrine system. Polyendocrinopathies have been described, and, so far, two diseases due to intestinal hormone excess (Zollinger-Ellison syndrome and the syndrome of watery diarrhea, hypokalemia and achlorhydria) elucidated. It seems likely that many more gastrointestinal endocrine diseases await discovery.