The localization of the vasoactive intestinal polypeptide (VIP) has been studied with immunohistochemistry and radioimmunoanalysis. VIP immunoreactivity is present in gastrointestinal nerves, which constitute a quantitatively important nerve population that may be intrinsic to the gut wall. VIP-immunoreactive neurons are also found within the ventromedial hypothalamus and give off processes that travel latteral to the third ventricle. Results of radioimmunoanalysis strongly indicate that the immunoreactive material represents true VIP. Thus VIP, at present a gastrointestinal hormone candidate, appears to represent a new neuronal peptide occurring in both the central and peripheral nervous system.