In the physiology of visual receptor cells, the processes intervening between the absorption of photons by a visual pigment and the change in membrane voltage (the receptor potential) are unknown. In general, receptor potentials are thought to be generated by changes in the ionic conductance channels. For veterbrate photoreceptors, the hypothetical intracellular excitatory transmitter is calcium. We propose to detect changes of intracellular free calcium directly using metallochromic dyes. On the other hand, for invertebrate photoreceptors, a change of intracellular free calcium has been proposed to mediate light-adaptation. We propose to determine the relationship between intracellular free calcium and the sensitivity of the photoreceptor using metallochromic dyes. We also propose to examine the generality of the hypohesis that calcium participates in adaptation of invertebrate photoreceptors by studying those of the squid.