As has often been observed in hypothalamic releasing factors and gastrointestinal hormones, the carboxy-terminal amide structure is a unique feature of peptides exhibiting hormonal or physiological activities. Although a variety of opioid peptides have hitherto been identified, such a C-terminal amidated species has never before been discovered in mammals. Here we present the first identification of a novel opioid octapeptide with a C-terminal amide structure, henceforth designated as 'adrenorphin', in human phaeochromocytoma tumour derived from adrenal medulla. The complete amino acid sequence of adrenorphin was determined by microsequencing and corresponds to the sequence of the first eight amino acids of peptide E which is derived from proenkephalin A. Adrenorphin has also been identified chromatographically in normal human and bovine adrenal medulla. Adrenorphin exhibits potent opioid activity in guinea pig ileum assay, suggesting a specialized physiological function.