The responses induced by G protein-coupled receptors wane during prolonged stimulation by an agonist, which is referred to as desensitization. Desensitization is a phenomenon that prevents cells from making unfavorable responses. There are two types of desensitization: homologous and heterologous desensitization. Desensitization of adenylyl cyclase is easily measured by in vitro enzyme assay. However, desensitization of adenylyl cyclase in intact cells can be measured by cellular cAMP accumulation. The receptor cDNA is subcloned into expression vectors, and then transfected into cells. When an expression vector does not contain a drug-resistant gene, the receptor should be co-transfected with a drug-resistant gene. In contrast with inhibitory receptors, desensitization of stimulatory receptors such as the β2-adrenergic receptor can be observed with a relatively low level of the receptors. Although transient transfection has several limitations, it provides a quick assay for analyzing receptor mutants, and the interaction between receptors, kinases, and β-arrestins.