In order to study the effect of N-methylation of a potent enkephalin analog, H-Tyr-D-Ala-Gly-Phe-NHNH-CO-CH₂CH₃, on analgesic activity, six new analogs were synthesized in which one or more of amino acid residues and the acyl-hydrazide constituting the tetrapeptide acyl-hydrazide were N-methylated. N-Methylation of both Tyr at position 1 and Phe at position 4 of the analog led to a derivative which was twice as potent as morphine. On the other hand, N-methylation of D-Ala at position 2, Gly at position 3 or NHNH-CO-CH₂CH₃ at position 5 markedly decreased the analgesic potency. Five analogs with a modified tyrosine residue at position 1 of the tetrapeptide acyl-hydrazide were also synthesized in order to assess the role of the N-terminal Tyr residue in the biological activity.