Previous studies of structure-activity of biphalin defined fragments which expressed the full biological potency of the parent compound. The most simple fragment was Tyr-D-Ala-Gly-Phe-NH-NH<--X, where X=Phe, but it also could be other hydrophobic amino acids. This paper presents data that replacement of the phenylalanine with a dansyl (X=DNS) groups gives an analogue (AA2016) that fully preserves the high affinity of the initial analogue for both mu and delta opioid receptors. In the tail flick test in rats, intrathecal injection of the compound produces strong antinociception, comparable to the parent biphalin. Because AA2016 contains a strong fluorescent group, it can be a very useful tool for prospective studies in vivo, including biological barrier permeability, tissue distribution, metabolism and receptor-ligand complex formation.