Résumé : In order to influence side-chain conformations and to increase the μ-agonist properties of the δ-selective opioid receptor δ-antagonist H-Tyr-Tic-Phe-Phe-NH2, residues Tyr1, Phe3 and Phe4 were replaced by their β-methyl-substituted stereoisomers. Synthesis of β-Me-Tyr was carried out in a stereoselective way. Incorporation of the modified amino acids was performed by SPPS. Receptor binding data and GPI and MVD bioassays were obtained for all stereoisomers, in general showing equal or slightly increased potencies. In the [(R,S)β-Me-Phe3]analogue, the introduction of the β-methyl substituent restores signal transduction. © 1995 ESCOM Science Publishers B.V.