The efficacy and potency of several endogenous opioid peptides were characterized in parallel to common opioid ligands in HEK cells stably expressing either µ, δ or κ opioid receptors. The endogenous peptides β-endorphin, dynorphin and endomorphin 1 showed the highest efficacies for activation of µ-opioid receptors. The efficacies of these ligands at µ-opioid receptors were equal to that of the highly potent analgesic fentanyl. In comparison met-enkephalin, morphine and the highly µ-selective opioid peptide DAMGO were partial agonists at µ- opioid receptors. At the δ-opioid receptors, fentanyl and DPDPE showed the highest efficacy. Compared to these ligands, both β-endorphin and the endogenous peptide met-enkephalin were partial agonists. The range of efficacies at κ-opioid receptors was much smaller. Fentanyl was again the most efficacious ligand while morphine, dynorphin, β- endorphin and the κ-selective ligand U69,593 all exhibited lower efficacies. The following conclusions can be made from this study. (1) The endogenous opioid peptide β-endorphin is a full agonist at the µ- opioid receptor; however it can also act as a partial agonist at δ-opioid receptors. At a two-fold higher concentration, β-endorphin can partially activate κ opioid receptors. (2) Endomorphin is a µ-selective full agonist. (3) Met-enkephalin is a δ-selective agonist, but relative to DPDPE, it is a partial agonist. (4) Dynorphin is a κ-selective peptide based on its almost 120-fold higher potency at κ vs. µ opioid receptors. However, at µ opioid receptors, dynorphin can act as a full agonist with one half the potency of β- endorphin.