The objective of the proposed research is to test our hypothesis that cyclic nucleotides, and other molecular species associated with cyclic nucleotides, including Ca ions, may be intermediary substances in the molecular chain of events between photon capture by the visual pigment and the decreased sodium ion conductance of the plasma membrane caused by illumination. The approach will be electrophysiologic and pharmacologic and the experimental material will be the photoreceptor organelles of the rods and cones of lower vertebrates. The physiologic effects of cyclic nucleotides, and related molecules, injected intracellularly into photoreceptor organelles will be monitored using microelectrodes. The effects of these molecules on Na ion conductance of the organelle's plasma membrane will be measured. Relations between the number of molecules injected and the ensuing physiological effects will be analyzed. The kinetics of the physiological effects caused by such injections will be compared to those caused by light to infer whether such injected substances are the actual intermediary substances. The size of the rod and cone compartments for these substances will be estimated.