Isolated rat tail arteries were perfused and vasoconstriction was evoked by electrical field stimulation (2 pulses at 1 Hz every 2 min). The vasoconstriction was depressed by DAGO (IC50 = 611 nM) and beta-endorphin (IC50 = 37 nM). Structuraly analogues and shorter fragments of beta-endorphin were also tested. beta-Endorphin and beta-endorphin-(1-26) were about equipotent whereas the beta-endorphin fragments 1-17, 1-16 and 6-31 were inactive. The potencies of beta-endorphin, beta-endorphin-(1-26), -(1-17) and -(1-16) were not changed in the presence of peptidase inhibitors. Structural analogues such as [D-Ala2]beta-endorphin or [Leu5]beta-endorphin had a somewhat lower potency than beta-endorphin itself. Naloxone 30 nM antagonized the effects of DAGO and beta-endorphin to a similar extent with dissociation constants 3.8 and 3.7 nM, respectively for the antagonist against the agonists. The results support the existence in the rat tail artery of a homogenous population of beta-endorphin-sensitive receptors which may belong to the epsilon-type.