A new family of cyclic opioid peptide analogues of the type H-Tyr-D-Xxx-Phe-Yyy-NH2 was obtained through amide bond formation between side chain amino and carboxyl groups of Orn (or Lys) and Asp (or Glu) residues substituted in positions 2 and 4 of the peptide sequence. Peptides were synthesized entirely by solid-phase techniques, and aside from the cyclic monomers, cyclization on the benzhydrylamine resin also produced side chain linked antiparallel cyclic dimers due to intersite reaction. In binding studies based on displacement of mu- and delta-opioid receptor-selective radiolabels from rat brain membranes the highly rigid cyclic monomer H-Tyr-D-Orn-Phe-Asp-NH2 (1) (containing a 13-membered ring) was shown to be one of the most selective mu-receptor ligands reported to date, whereas the corresponding cyclic dimer, (H-Tyr-D-Orn-Phe-Asp-NH2)2 (1a), was nonselective. The difference in receptor selectivity observed between 1 and 1a is a consequence of the different conformational constraints present in the cyclic monomer and dimer. In contrast to 1, the conformationally less restricted cyclic analogue H-Tyr-D-Lys-Phe-Glu-NH2 (3) (15-membered ring) showed no receptor preference. Qualitatively similar potency relationships were observed in the guinea pig ileum (GPI) and mouse vas deferens (MVD) bioassays. However, in the case of analogues 1 and 3 discrepancies observed between potencies determined in the mu-receptor-representative GPI bioassay and in the mu-receptor-selective binding assay seemed to indicate that the conformational constraint present in these compounds may produce an "efficacy" enhancement. Corresponding analogues containing an Asp (or Glu) residue in the 2-position and an Orn (or Lys) residue in the 4-position showed similar selectivity relationships, but better agreement between bio- and binding assay data. These results indicate that incorporation of various conformational constraints into opioid peptides permits manipulation of both receptor selectivity and efficacy.