The [(35)S]GTPgammaS binding assay to measure G protein activation following agonist binding to G protein-coupled receptors (GPCRs) remains a powerful molecular technique to substantiate traditional pharmacological values of potency, efficacy, and affinity. The method described uses membrane preparations of the alpha(2A)-adrenergic receptor and purified G protein subunits expressed in Sf9 cells, reconstituted into a functional signaling system. This technology is generic and could be used with other GPCRs to demonstrate initial signaling events following receptor activation. Agonist-stimulated [(35)S]GTPgammaS binding is measured in a 96-well plate format using scintillation counting.