G protein-coupled receptors (GPCRs) regulate a wide variety of physiological functions in response to structurally diverse ligands ranging from cations and small organic molecules to peptides and glycoproteins. For many GPCRs, structurally related ligands can have diverse efficacy profiles. To investigate the process of ligand binding and activation, we used fluorescence spectroscopy to study the ability of ligands having different efficacies to induce a specific conformational change in the human beta2-adrenoceptor (beta2-AR). The 'ionic lock' is a molecular switch found in rhodopsin-family GPCRs that has been proposed to link the cytoplasmic ends of transmembrane domains 3 and 6 in the inactive state. We found that most partial agonists were as effective as full agonists in disrupting the ionic lock. Our results show that disruption of this important molecular switch is necessary, but not sufficient, for full activation of the beta2-AR.