1. An new enkephalin analog of the formula (I), EQU Tyr-X-Gly-Phe-Y (I) wherein Tyr, Gly and Phe represent L-tyrosyl, glycyl and L-phenylalanyl residues, respectively, X is a glycyl group or a D-.alpha.-aminocarboxylic acid residue with a lower alkyl, lower thioalkyl or phenyl-(lower)-alkyl side chain, and Y is the residue of an L, D or DL-.alpha.-aminophosphonic acid or L, D or DL-.alpha.-aminosulfonic acid, each having a lower alkyl side chain, or a salt thereof.