We have been studying the light activated phosphodiesterase in vertebrate rod outer segments. Recently we have identified a light activated GTPase which appears to function as a component in the activation sequence. By studying the relationships of rhodopsin, the light activated GTPase and the light activated phosphodiesterase, we are able to show that light activation of phosphodiesterase is dependent on the presence of both GTP (or one of its analogues) the GTPase and bleached rhodopsin. However, the light activation of the PDE, while dependent on the presence of bleached rhodopsin, is independent of PDE. These data, obtained from simple recombination experiments, suggests that the flow of regulatory influences proceeds from photon capture through the activation of the GTPase and finally terminates in the formation of an active outer segment PDE. Out planned studies encompass further efforts at purification and study of interactions between the components of the light sensitive activation cascade for PDE activation.