Tomita has shown by electrophysiological measurements that the photoreceptors of the vertebrate retina are depolarized (excited) by darkness and hyperpolarized (inhibited) by light. Excitation is accompanied by an increase, and inhibition by a decrease, in the sodiumion permeability of the receptor cell. The retinal-rod outer segments of the frog contain an adenyl cyclase that is active in darkness and inactivated by light. This cyclase has a specific activity ten-times higher than the activity in previously described tissues. It is suggested that cyclic AMP is an intermediate in the light- and dark-induced changes in sodium permeability in the photoreceptor cell.