The steps that intervene between the absorption of a photon and the flow of electrical current across the photoreceptor membrane will be studies in the ventral photoreceptor of Limulus. Using intracellular recording techniques and voltage-clamp methods, the kinetics and chemical nature of the intervening reactions will be explored. Previous experiments suggest that calcium plays a key role in the process. The effects of calcium, temperature, and of the two together on response latency, waveform, amplitude, and quantum efficiency will be examined to provide a hypothesis for the sequence of intervening reactions and to suggest their chemical nature. Calcium also plays a role in light adaptation. However, previous experiments suggest that calcium does not completely control light sensitivity and that other unknown factors are involved as well. With relatively weak adapting lights, light adaptation appears to occur without a measurable change in the concentration of intracellular ionized calcium. The mechanism of light adaptation will be explored with attention to both the calcium dependent and calcium independent processes involved.