In mammalian retina photoreceptor cells, light-dependent rhodopsin phosphorylation and dephosphorylation play pivotal roles in the regulation of its phototransduction cascade. To elucidate the biological and pathological aspects of rhodopsin phosphorylation and dephospho-rylation, we initially studied sites of rhodopsin phosphorylation in vitro and in vivo using biochemical assays using mass spectrometry or radioisotope, and found serine 334, 338 and 343 as major sites. Furthermore, rhodopsin phospho-rylation at these sites is unique and regulated differently under different illumination conditions. Recently we have developed a new method for determining rhodopsin phosphorylation and dephosphorylation kinetics at each respective site by utilizing specific antibodies toward each phosphorylation site. Interestingly, this assay revealed rhodopsin dephosphorylation was commonly delayed in animal models of retinal degeneration, suggesting slow kinetics of rhodopsin dephosphorylation may cause retinal dysfunction leading to its degeneration.