We have shown by intravitreal injection of [3H]mevalonolactone that a 65 kDa protein in rat photoreceptors is posttranslationally modified by farnesylation. We further identified this 65 kDa prenylated protein as rhodopsin kinase based on its affinity for photolyzed rhodopsin and its ability to autophosphorylate in the presence of [gamma-32P]ATP. The farnesylation of rhodopsin kinase may be important for correctly targeting this enzyme to the photoreceptor outer segments, allowing it to phosphorylate photolyzed rhodopsin efficiently.