Evaluation of hypotheses concerning the role of opioid peptides in the human adrenal is handicapped by the lack of information concerning the nature of these peptides. We studied the content of opioid peptides in whole adrenal tissue using several RIAs, including one which cross-reacts with all opioid peptides tested. Opioid peptides were localized to granules which behaved like chromaffin granules on crude sucrose density separation. β-Endorphin immunoreactivity was a minor component, which was found principally in the form of β-endorphin-(1–31). The majority of the remaining peptides probably were products of proenkephalin. The human postmortem tissue differed from bovine tissue in that the major accumulating products of this precursor were the size of the enkephalins and their small congeners, and not the intermediate-sized (mol wt, ∼1500–4000) peptides that predominated in bovine tissue. We also found evidence of the presence of the 25-amino acid complex opioid, peptide E, in human tissue.