Analogues of angiotensin II and III (ANG II and ANG III) in which the tyrosine and/or phenylalanine residues were substituted have been synthesized by the solid-phase method and purified by (carboxymethyl)cellulose chromatography and reversed-phase HPLC. The antagonist and agonist potencies of these peptides were determined in the rat isolated uterus assay. [Sar1,Tyr(Me)4]ANG II, [Tyr(Me)3]ANG III, [Sar1,D-Trp4]ANG II, [D-Trp3]ANG III, [Sar1,D-Trp8]ANG II, [D-Trp7]ANG III, [Sar1,Tyr(Me)4,Ile8]ANG II, [Tyr(Me)3,Ile7]ANG III, [Sar1,D-Trp4,Ile8]ANG II, [D-Trp3,Ile7]ANG III, [Sar1,Tyr(Me)4,D-Trp8]ANG II, and [Tyr(Me)3,D-Trp7]ANG III had antagonist activities (pA2) respectively of 8.1, less than 6, less than 6, less than 6, (7.7), (6.7), 7.2, less than 6, less than 6, less than 6, 7.1, and less than 6. The agonist activity of each peptide was less than 0.1% of that of ANG II. Analogues in which only the Phe residue was substituted were not readily reversible in the bioassay, whereas analogues in which only the Tyr residue or both the Tyr and Phe residues were substituted were reversible antagonists. Peptides that were twice substituted had lower antagonist activities than peptides having a single aromatic residue substitution. Substitution of the Tyr residue in ANG II, but not ANG III, provides a new route for the synthesis of potent and competitive angiotensin antagonists. Differences in the biological properties of ANG II and ANG III analogues substituted at the Tyr residue suggest different binding/conformation requirements for the two endogenous ligands at angiotensin receptors in smooth muscle.