The molecular forms of opioid peptides in human adrenal have not been well characterised. These peptides are predominantly derived from the proenkephalin A precursor, which has the sequence of Met-enkephalin(Arg6,Phe7) as its carboxyl terminus. We have looked in the present study at the subcellular distribution and the molecular form of immunoreactivity to this sequence in post-mortem human adrenal medulla and in phaeochromocytoma. In the human adrenal homogenates, the immunoreactivity distributes on a sucrose gradient in a manner consistent with localisation in chromaffin granules. On chromatography, the immunoreactivity from adrenal medulla is predominantly in the heptapeptide form; the intermediate (3000-4000) molecular weight material is only a minor component of immunoreactivity, in contrast to bovine tissue extracts where this is the major form of immunoreactivity. In the phaeochromocytoma extracts, the heptapeptide sequence again predominates over a minor amount of intermediate sized material. The results are discussed in terms of post-mortem changes, precursor processing and the function of the adrenal medulla.