In contrast to other families of cell surface receptors, like tyrosine kinase receptors, for which dimerization is an integral part of the activation process, G-protein-coupled receptors (GPCRs) were thought, until recently, to function as monomeric units. However, a growing body of evidence indicates that GPCRs could exist and be active as oligomeric complexes. Because they are major pharmacological targets, their existence as homo- or hetero- oligomers could have important implications for the development and screening of new drugs. The major evidences supporting the idea of GPCR oligomerization come from indirect biochemical or pharmacological experiments. Here we report, using traditional co-immunoprecipitation methods, the existence of differentially epitope-tagged beta2-adrenergic receptor (beta2AR) oligomers in mammalian HEK-293 cells. Moreover, we validate the existence of receptor oligomers in living cells by a new Bioluminescence Resonance Energy Transfer (BRET) technique. Our results clearly demonstrate the presence of constitutive beta2AR oligomers in living cells that can be modulated by the selective adrenergic agonist isoproterenol, suggesting a pertinent physiological role for GPCR oligomerization.