Abstract: Five derivatives of 4-methoxycarbonyl fentanyl were synthesized, four of themcarry in the molecules some radicals that may alkylate opiate receptors. Their analgesic activities weremeasured and the irreversible inhibitory effects of all compounds on the electrically elicited contractionof MVD were investigated. The analgesic test showed that the compounds in this series possess analgesicactivity with typical morphine--like action. The isolated tissue experiment indicated that these newcompounds were all capable of binding with opiate receptors, and that compounds 2, 4 and 5 weredetermined to be a new irreversible ligand of opiate receptors.