Tyr-D-Arg-Phe-beta Ala-NH2 (I) and its six fluorinated analogs were synthesized. Their opioid receptor binding properties were examined in vitro and their analgesic activity in vivo using the mouse writhing test. It was found that I was one of the most selective and potent mu-receptor agonists reported to date. [Tyr(2F)1](VI) and [Tyr(3F)1](V) derivatives of I showed similar biological properties to those of I. Since these peptides resist enzymatic degradation, it is expected that they are excellent reagents for the studies of function of mu-receptor-mediated biological properties in vivo and in vitro.