The electrical signals of vertebrate photoreceptors are generated by modulation of an inward current of sodium ions at the outer segment. The mechanism by which Na enters the outer segment is not known, and there is little solid evidence about how light controls the conductance of the pores or carrier molecules that mediate the Na influx. We propose to examine the properties of the light-sensitive conductance and the mechaism of its control, using patch clamp and suction electrode recording, optical measurements, and injection of substances into isolated rods. We intend to approach the following questions: 1) What is the electrical conductance, temperature dependence and kinetic behaviour of the light-sensitive "channel" in the surface membrane of the rod outer segment? 2) How does the channel respond to possible internal transmitters such as CA ions and cyclic GMP? 3) How does the internal concentration of free Ca change during a rod's response to light? 4) What is the origin of the continuous component of the electrical dark noise of rods? 5) What is the magnitude and power spectrum of the dark noise of cones, and how is the noise generated? 6) In the outer segment of a dark-adapted rod is there a longitudinal gradient of internal Na concentration? Is a Na gradient responsible for the different kinetics of transduction at the tip and base of the outer segment?