Heterotrimeric G protein-mediated signal transduction is a dynamically regulated process with the intensity of signal decreasing over time despite the continued presence of the agonist. This phenomenon, referred to as agonistmediated desensitization, involves phosphorylation of the receptor by two classes of enzymes. The first class is comprised of the second messengerregulated kinases, such as c-AMP dependent protein kinase A and protein kinase C. The second class includes the G protein-coupled receptor kinases (GRKs). At least seven members of the GRK family have been identified. These include rhodopsin kinase (GRK 1alpha and beta); two forms of beta-adrenergic receptor kinase: GRK 2 (betaARK, betaARK1) and GRK 3 (betaARK2); IT-11 (GRK 4); GRK 5; GRK 6; and GRK 7. Phosphorylation of receptors by GRKs appears to be strictly dependent on the receptor being in its agonist-activated state.