[en] Light initiates a series of biochemical and biophysical changes in vertebrate rod outer segments (ROS). A variety of studies have indicated that several discrete biochemical steps occur between photon capture and the subsequent change in rod membrane voltage. Cyclic GMP is a possible candidate for a transmitter molecule in the photon transduction process. Electrophysiological studies have indicated that iontophoresis of cyclic GMP into dark adapted rods is accompanied by a reduction in plasma membrane voltage (33). Characterization of rod phosphodiesterase (PDE) indicates a large turnover number and rapid rates of activation following illumination, which are compatible with a role of cyclic GMP in the (approximately 100 msec) time span for visual excitation. An attractive hypothesis assigns a transmitter role to disc Ca²⁺ and is supported by many intriguing experiments. There is, however, still disagreement concerning the effect of cyclic GMP on rod Ca²⁺ dynamics. These and other studies have suggested that light regulation of PDE plays an important role either in support of transduction or in support of light adaptation or the regulation of photoreceptor sensitivity. Work has continued to focus on elaboration of the mechanism for the light activation of ROS PDE. In the following section, the authors briefly review the current understanding of the activation mechanism and describe some newer findings now emerging from our ongoing studies