An octapeptide agonist of μ-opioid receptors; selectively binds to μ-opioid receptors over κ- and δ-opioid receptors (Kis = 0.12, 0.248, and 2.65 nM, respectively); increases the latency to paw licking in the hot plate test in mice at 6.1 nmol/animal, i.c.v.; induces respiratory depression in mice and rabbits (ED50s = 6 and 71.1 nmol/animal, respectively)