Adrenorphin is the first C-terminally amidated form of opioid peptide isolated from human pheochromocytoma tumor and is considered to be generated out of proenkephalin A by unique processing. We have developed a highly specific and sensitive radioimmunoassay for adrenorphin as well as for PH- 8P , whose structure and processing are similar to adrenorphin . Prior to the measurement of both peptides in rat brain, immunoreactive adrenorphin and PH- 8P were verified to be identical with their individual authentic peptides by high performance liquid chromatography. Here we have determined the distribution of adrenorphin in rat brain by radioimmunoassay, and compared it with that of PH- 8P . The regional distribution of adrenorphin was found to be quite different from that of other endogenous opioid peptides including PH- 8P . The highest concentration of adrenorphin was found in the olfactory bulb. These results suggest that adrenorphin is generated by a specific processing mechanism and may have a unique physiological function distinct from that of known opioid peptides. In addition, we identified adrenorphin also in human and bovine adrenal medullas.