Adrenorphin is the first C-terminally amidated form of opioid peptides isolated from human pheochromocytoma tumor and is considered to be generated out of proenkephalin A by unique processing. By the highly specific and sensitive radioimmunoassay (RIA) procedure utilizing the antiserum against adrenorphin, combined with high performance liquid chromatography (HPLC), immunoreactive adrenorphin in rat brain was verified to be identical with its authentic peptide. It has been revealed that adrenorphin immunoreactivity distributes widely in rat brain but in the unique pattern distinct from those of other endogenous opioid peptides. Note that immunoreactive adrenorphin was most concentrated in the olfactory bulb, and appreciably in the hypothalamus and striatum. Furthermore, immunohistochemical study has revealed that adrenorphin-immunoreactive structures in hypothalamic region of rat were localized in the neurones of the arcuate nucleus. In addition, adrenorphin-immunoreactive fibre plexus was found in the various regions of the hypothalamus, such as median eminence, periventricular zone and paraventricular nucleus. These indicate that adrenorphin may have a unique physiological function.