Our molecular modeling studies suggested that the conformational effects of the "cystine-line" residue Pmp1-Cys6 on the cyclohexapeptide ring of the vasopressin antagonist [Pmp1,D-Phe2,Val4]AVP might be mimicked by substitution of D-aminoadipic acid at position 6 and cyclization of its side-chain carboxyl to the alpha-amine of residue 2. The peptide was prepared with DL-aminoadipic acid, and following cyclization, the two diastereomeric peptides were separated and purified by preparative high-performance liquid chromatography. The structure of each was confirmed by amino acid analysis and fast atom bombardment mass spectrometry. The chirality of the aminoadipic acid residue of each peptide was determined by chiral gas chromatography. The circular dichroism spectrum of each peptide was run and compared with the appropriate agonist and antagonist peptide standards. These peptides demonstrated in vitro poor V2-receptor affinity and an inability to inhibit or stimulate vasopressin-induced adenylate cyclase formation, suggesting that they lack one or more key features of the agonist/antagonist pharmacophore.