The angiotensin AT1 receptor mediates numerous physiological actions of the octapeptide hormone, angiotensin II, in its cardiovascular and other target tissues. The binding of angiotensin II to the AT1 receptor is dependent on both intramembrane and extracellular regions of the receptor molecule. Non-peptide antagonists that block angiotensin binding and action interact exclusively with residues located within the intramembrane binding pocket of the AT1 receptor. However, peptide ligands also interact with extracellular residues to form additional bonds that stabilize their binding to the receptor. Here, László Hunyady, Tamás Balla and Kevin Catt describe how these and other studies have shown that interaction of ligands with residues in the intramembrane binding pocket is the conserved mechanism required for agonist activation of G protein-coupled receptors.