Recent studies on the highly potent and selective δ-opioid agonists demenkephalin (Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2) and deltorphin I (Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH2) suggested that key structural features necessary for specific targetting to the δ-opioid receptor are located within the C-terminal halves of these naturally occurring heptapeptides. To investigate the contribution of aspartic acid 4 residue in deltorphin I and aspartic acid 7 residue in dermenkephalin to the δ-addressing ability of the C-terminal ends, fourteen analogs were synthesized and assessed for their ability to bind to μ and δ-opioid receptors in rat brain membrane homogenates. Results showed that i/ although the tetrapeptide C-terminus of dermenkephalin and deltorphin I differ in amino acid composition, they play a similar role in specifying correct addressing of these peptides to the δ-receptor, ii/ the negatively charged side chain of aspartic acid 4 residue in deltorphin I and aspartic acid 7 residue in dermenkephalin is not involved in binding contact at the δ-receptor site, nor in maintaining a δ-bioactive folding of the peptides, iii/ these side chains are, in contrast, functionnally or structurally required to confer high δ-selectivity by preventing μ-site recognition and / or binding.