Neuropeptide FF(NPFF)is known to be an endogenous opioid-modulating peptide and mediate a variety of biological actions[1-2].To date,the abilities of NPFF and the related peptides to inhibit morphine-induced antinociception have been reported[3].However,very few studies directly examined the functional interaction between NPFF and the endogenous opioid agonists.Interestingly,endomorphin-1(EM-1)and endomorphin-2(EM-2),were reported to be the selective agonists of μ-opioid receptor and have important roles in pain managements[4-5].In our previous study,we found that NPFF inhibits the acquisition and expression of EM-2 CPA[6].