A radioreceptor assay (RRA) method is described to detect opioid peptides that uses a canine limbic system P2receptor preparation and3H-methionine enkephalin as the displaced ligand. In our laboratory,3H-ME is a useful probe to detect the presence of, and to facilitate the description of, opioid peptides in biological extracts. Those opioid receptor-active peaks are then studied subsequently in greater detail with immunoassay and mass spectrometry methods. A variety of peptide ligands including leucine enkephalin, beta-endorphin, Tyr-D-Ala-Gly-NMe-Phe-Gly-OL (“DAGO”), Tyr-D-Ser-Gly-Phe-Leu-Thr (“delta-receptor peptide”), and several dynorphins (1-7, 1-8, 1-9, 1-10, 1-17, and B) were used to further characterize the molecular specificity of that RRA system.