Rhodopsin kinase and the beta-adrenergic receptor kinase (beta ARK) catalyse the phosphorylation of the activated forms of the G-protein-coupled receptors, rhodopsin and the beta 2-adrenergic receptor (beta 2AR), respectively. The interaction between receptor and kinase is independent of second messengers and appears to involve a multipoint attachment of kinase and substrate with the specificity being restricted by both the primary amino acid sequence and conformation of the substrate. Kinetic, functional and sequence information reveals that rhodopsin kinase and beta ARK are closely related, suggesting they may be members of a family of G-protein-coupled receptor kinases.