The duration of photoresponses in vertebrate rods and cones is controlled at the level of GTP hydrolysis by a GTPase accelerating protein (GAP) whose catalytic core is provided by RGS9-1. RGS9-1 is in turn regulated by phosphorylation on serine 475, in a reaction that is dependent on Ca2+. In living mice, the level of phosphorylation at this site is reduced by light. Thus RGS9-1 phosphorylation provides a potential mechanism by which light-regulated changes in intracellular [Ca2+] may feed back on phototransduction through effects on the lifetime of activated G protein and cGMP phosphodiesterase.