The potency of a number of opioid agonists have been determined on three different bioassays, the isolated guinea-pig ileum (GPI), the mouse vas deferens (MVD) and the rat vas deferens (RVD). With the exception of beta-endorphin, compounds with opioid activity on the GPI and the MVD exhibited considerably less or no activity on the RVD; the opiate receptor in this latter tissue being thus characterized as beta-endorphin selective ('epsilon-receptor'). A system of classification of opioids according to their relative relative affinities for particular opiate receptor types is given.