We describe the design, the conformational behavior, and the biological activity at the mu-opioid receptor of new morphiceptin analogues. In these analogues a recently described dipeptide mimetic structure replaces both the N- and the C-terminal Xaa-Pro dipeptide of morphiceptin. Conformational investigation on the most active analogue, compared to the parent peptide, indicates a high degree of structural tolerance within the mu-opioid receptor binding site. In fact, our results indicate that only the location and the relative orientation of the side chains of the aromatic pharmacophoric residues represent the indispensable structural features for mu-receptor binding. To reach such topological arrangement, opioid peptides can adopt different conformations and configurations. In particular, opioid peptides bearing a proline residue as spacer between the two aromatic residues can adopt, in the active state, both cis and trans configurations at the Tyr(1)-Pro(2) amide bond, each of them with the appropriate backbone and side chains orientations.