The light peak (LP) is a slow increase in the standing potential of the eye and has been attributed to a depolarization of the basal membrane of the retinal pigment epithelium (RPE). We tested effects of different external calcium ion concentrations ([Ca++]0) on the LP in the arterially perfused cat eye. An increase in Ca++ activity by 3.2 mM depressed the LP by 85-90% of its amplitude under control conditions. In contrast the other components of the light response (c-wave, fast oscillation, and second c-wave) did not change. The small increase in [Ca++]0 had insignificant effects on the amplitude-intensity plots of the a- and b-waves of the electroretinogram. Elevated [Ca++]0 had its effect only during onset of light and not after the LP was once initiated. The effects were reversible. Since there is a very high [Ca++] concentration in the RPE cells and the pigment granulas may release Ca++ during illumination (Hess 1975), our results suggest that this ion plays a role during the initiation of the LP.