Light absorption by photoreceptor rod outer segments (ROS) leads not only to spectral and structural changes in the rhodopsin molecule but also to the activation of several enzymatic activities including GTPase. The mechanism of this effect is not known; however, in all cases where the action spectrum of light-induced enzyme activation has been measured (see, for example, refs 4, 8), it matched the absorption spectrum of rhodopsin. This suggests that bleaching of rhodopsin is the primary step in enzyme activation, and that some light-induced changes in the molecular interaction of the enzymes with rhodopsin, the major intrinsic ROS membrane protein, should be involved. I have indeed found that light induces profound changes in the interation of rhodopsin kinase, GTPase (reported here), and other proteins with the photoreceptor membrane. These changes are reversible in the dark, are strongly influenced by GTP, and are thought to be involved in the regulation of enzyme activity by light. The light-induced binding of the GTPase and its subsequent elution with GTP was used to purify this enzyme.