The interaction of dermorphin with different peripheral opioid receptor subtypes was investigated in vitro, using the guinea pig ileum as representative tissue for mu, the mouse vas deferens for delta, the rabbit vas deferens for kappa and the rat vas deferens for epsilon. The effect of dermorphin on each tissue preparation was compared with that of selective mu, delta, kappa epsilon agonists respectively morphine, met-enkephalinamide, ethylketocyclazocine and camel beta-endorphin. Antagonism with naloxone was also tested and calculated as Ke. It is concluded that dermorphin mainly interacts with the mu receptors, although it also binds to epsilon receptors; the interaction with delta receptors is questionable, and the kappa receptors are unaffected.