Endomorphin-2 (Tyr-Pro-Phe-Phe-NH(2)) is a putative endogenous mu-opioid receptor ligand. To develop potent analgesics with less side effects related to it, we used the methods of dimerization and C-terminal modification. Through dimerization we got the 'balanced agonists' with potent analgesic activity and we have developed the structure-activity relationship between the selectivity and the distance of the two tyrosine pharmacophores. Modification at the C-terminal increased the selectivity of endomorphin-2 to mu-opioid receptor with binding affinity conserved.