The electrical responses which occur when single photons are absorbed by Limulus ventral photoreceptors will be used to formulate a kinetic model for phototransduction. This study will characterize the latency between photon absorption and response initiation, the characteristics of the response, called the quantum bump, and the mechanism by which photon absorption is transmitted to a coupled cell. A new method based on Wiener least squares prediction theory will be used to determine the latency. From studies on latency, in which calcium and temperature will be used as probes, a better understanding of the mechanism of photoexcitation, the number of reactions involved, their rate constants and activation energies, can be obtained. Analysis of the parameters which describe quantum bumps and bump variability can yeild information about ionic channel kinetic mechanisms in photoreceptor plasma membranes. Analyzing bump parameters as a function of intracellular calcium ion concentration may be used to determine calcium's role in membrane channel gating. Anatomical and preliminary electrophysiological studies suggest that photoreceptors in the ventral eye of Limulus are coupled via gap junctions. Examination of electrical responses to photic and electric stimulation of coupled cells will elucidate the coupling mechanism. Since calcium also has a marked effect on coupling this study will attempt to separate its junctional effects from its intracellular effects on the photoexcitation mechanism and plasma membrane channels and pumps.