Recent studies on the nucleotide sequence of cloned DNA complementary to mRNA for preproenkephalin from bovine adrenal medulla and human pheochromocytoma have revealed that this precursor molecule contains four copies of methionine-enkephalin (Met-Enk) and one copy each of leucine-enkephalin (Leu-Enk), methionine-enkephalin-Arg6-Gly7-Leu8 (Met-Enk-Arg6-Gly7-Leu8) and methionine-enkephalin-Arg6-Phe7 (Met-Enk-Arg6-Phe7). We have demonstrated the existence of Met-Enk-Arg6-Gly7-Leu8 together with Met-Enk, Leu-Enk and Met-Enk-Arg6-Phe7 in human gastric antrum, using high performance liquid chromatography (HPLC) coupled with radioimmunoassays for these opioid peptides. The ratio of molar concentrations of these peptides in human gastric antrum is similar to the ratio of these peptides contained in preproenkephalin. Furthermore, gel filtration studies on Sephadex G-50 showed that most of immunoreactivities of these peptides were eluted at the elution position of each synthetic peptide without any detectable immunoreactivities at high molecular weight positions. These results indicate the presence of Met-Enk-Arg6-Gly7-Leu8 together with Met-Enk, Leu-Enk and Met-Enk-Arg6-Phe7 in human gastric antrum and further suggest that these opioid peptides are derived from the same preproenkephalin as that in the adrenal medulla and that the processing of preproenkephalin is almost complete in the gut.