A key reaction in the inactivation of rhodopsin is its phosphorylation by rhodopsin kinase. In recent years, extensive studies related to rhodopsin kinase function and enzymatic properties were carried out. Rhodopsin kinase is a Ser/Thr protein kinase and a member of the G protein-coupled receptor kinases sub-family (GRKs) which consists of six recently identified members. Photolyzed rhodopsin is phosphorylated by rhodopsin kinase sequentially, with the first phosphate transferred preferentially to Ser-338, and subsequent phosphates transferred to Ser-343 and Thr-336. The binding of arrestin to the receptor, and reduction of the photolyzed chromophore all-trans-retinal to all-trans-retinol limits physiologically significant phosphorylation at no more than three sites (H. Ohguro, R.S. Johnson, L.H. Ericsson, K.A. Walsh and K. Palczewski, Biochemistry, 33 (1994) 1023). A similar phosphorylation reaction is implicated in most, if not all, G protein-coupled receptors during their desensitization.