The status and use of transmembrane, extracellular and intracellular domains in oligomerization of heptahelical G-protein coupled receptors (GPCRs) are reviewed and for transmembrane assemblies also supplemented by new experimental evidence. The transmembrane-linked GPCR oligomers typically have as the minimal unit an asymmetric similar to 180 kDa pentamer consisting of receptor homodimer or heterodimer and a G-protein subunit heterotrimer. With neuropeptide Y (NPY) receptors, this assembly is converted to similar to 90 kDa receptor monomer-G complex by receptor and G agonists, and dimers/heteropentamers are depleted by neutralization of Gi subunits by pertussis toxin. Employing gradient centrifugation, quantification and other characterization of GPCR dimers at the level of physically isolated and identified heteropentamers is feasible with labeled agonists that do not dissociate upon solubilization. This is demonstrated with three neuropeptide Y (NPY) receptors and could apply to many receptors that use large peptidic agonists.