Conformationally restricted deltorphin analogues were synthesized either through incorporation of cyclic phenylalanine analogues in position 2 or 3 of the peptide sequence or through various side chain-to-side chain cyclizations. Compounds were tested in mu-, delta-, and kappa-receptor selective binding assays and in the guinea pig ileum (GPI) and mouse vas deferens (MVD) bioassays. Replacement of Phe3 in [D-Ala2]deltorphin I with 2-aminoindan-2-carboxylic acid (Aic) or L- or D-2-aminotetralin-2-carboxylic acid (Atc) resulted in agonist compounds which retained the high delta receptor selectivity of the parent peptide. Substitution of a tetrahydroisoquinoline-3-carboxylic acid (Tic) residue in the 2-position of [D-Ala2]deltorphin I and of [Phe4,Nle6]deltorphin produced a partial delta agonist, H-Tyr-Tic-Phe-Asp-Val-Val-Gly-NH2, and a pure delta antagonist, H-Tyr-Tic-Phe-Phe-Leu-Nle-Asp-NH2, respectively. The latter antagonist displayed high delta selectivity (Ki mu/Ki delta = 502) and was a potent antagonist against selective delta agonists in the MVD assay (Ke congruent to 10 nM). Various [D-Ala2]-deltorphin I analogues cyclized between the side chains of Orn (or Lys) and Asp (or Glu) residues substituted in positions 2 and 4, 4 and 7, and 2 and 7 were essentially nonselective. Comparison with corresponding N-terminal tetrapeptide analogues revealed that the C-terminal tripeptide segment in the deltorphin heptapeptides made a crucial contribution to delta affinity and delta selectivity in the case of the agonist peptides but not in the case of the antagonist.