series of analogues of BQ-123 (1), a potent cyclic pentapeptide endothelin A receptor antagonist, with amino acids linked to the side-chain of the Pro residue via an ester linkage was synthesized. All analogues synthesized exhibited potent endothelin A receptor binding affinity similar to that of 1. Of the synthesized analogues, the Lys, Arg and N alpha,N epsilon-dimethyllysine analogues, 9d-f, exhibited