The potent opioid peptide beta-endorphin is found in the brain and pituitary with two related fragments, beta-endorphin-(1-27) and beta-endorphin-(1-26). The fragments retain substantial opioid-receptor binding activity but are virtually inactive analgesically. beta-Endorphin-(1-27) inhibits beta-endorphin-induced and etorphine-induced analgesia when coinjected intracerebroventricularly into mice. Antagonism by competition at the same site(s) is suggested from parallel shifts of the dose-response curves of etorphine or beta-endorphin in the presence of beta-endorphin-(1-27). Its potency is 4-5 times greater than that of the opiate antagonist naloxone. beta-Endorphin-(1-26) does not antagonize the antinociceptive action of etorphine or beta-endorphin in doses up to 500 pmol per animal.