To understand the effect of the replacement of Tyr residue at position 1 in opioid peptides by 2, 6-dimethyl-Tyr (Dmt) on the biological property, chiral (D or L) Dmt¹ analogs of Leu-enkephalin (Enk) and Tyr-D-Arg-Phe-βAla-NH₂ (YRFB) were synthesized and their enzymatic stabilities, in vitro bioactivities and receptor binding affinities compared with those of parent peptides. [L-Dmt¹]Enk (1) exhibited 4-fold higher stability against aminopeptidase-M and possessed dramatically increased activities in guinea pig ilium (GPI) (187-fold) and mouse vas deferens (MVD) (131-fold) assays, and in rat brain receptor binding assays (356-fold at μ receptor and 46-fold at δ receptor) as compared to Enk. [L-Dmt¹]YRFB (3) also exhibited increased activties in GPI (46-fold) and MVD (177-fold) assays, and in the binding assays (69-fold at μ receptor and 341-fold at δ receptor) as compared to the parent peptide. [D-Dmt¹]Enk (2) and [D-Dmt¹]YRFB (4) exhibited activities with diminished or lesser potency than the parent peptide in all assays. These results indicate that there is a tendency for μ affinity to be enhanced more than δ affinity with introduction of L-Dmt into δ ligand peptide (Enk), and for δ affinity to be enhanced more than μ affinity in case of μ ligand peptide (YRFB), resulting in reduced receptor selectivities at the receptors.