Endomorphin 2 is a potent, selective endogenous ? opioid receptor agonist. The physiological function of endomorphin-2 as a neurotransmitter or neuromodulator may cease through the rapid enzymatic process in the synapse of brain, as for other neuropeptides. Some results indicate that the administration of the endogenous ?-opioid ligand endomorphin-2, but not endomorphin-1, into the brain produces an antitussive effect via mainly naloxonazine-insensitive ?-opioid receptors, namely ?2-opioid receptors and partially kappa-opioid receptors.