1. The light peak is a large light-induced change in the DC potential across the eye (standing potential) that reaches its maximum in 5-13 min in mammals. The light peak of the intact cat eye was studied in order to define its cellular origin and stimulus-response characteristics. Direct-coupled recordings were made with a vitreal electrode and also with intraretinal and intracellular micro-electrodes. Light peaks were generally evoked with 300 sec periods of diffuse white illumination.2. Micro-electrode recordings made in the subretinal space just outside the apical membrane of the retinal pigment epithelium (r.p.e.) showed that the light peak was a change in trans-epithelial potential. No component was generated in the neural retina.3. Intracellular recordings from r.p.e. cells showed that the change in trans-epithelial potential resulted from a depolarization of the basal membrane (facing the choroid). This depolarization came after the hyperpolarization of the apical membrane that gave rise to the r.p.e. component of the c-wave of the e.r.g.4. The light peak amplitude at a constant retinal illumination was nearly linear with stimulus duration over the range 15-180 sec, and saturated at about 300 sec. The time-to-peak remained nearly constant at about 300 sec over this range. Large light peaks could be evoked with flashes as short as 10 sec if the retinal illumination was several log units above rod saturation.5. When stimulus duration was held constant at 300 sec, light peak amplitude was graded with illumination over a wide range, from 3 log units below to 2 log units above rod saturation. The threshold of the light peak was below that of the e.r.g. and only about 1.5-2.5 log units above the absolute threshold of the most sensitive ganglion cells. The increase of light peak amplitude above rod saturation was not due primarily to cones.6. The trans-epithelial light peak had an unusual dependence on stimulus area, being at least twice as large in response to diffuse light as it was in response to a large spot (10 deg diameter) of the same retinal illumination.7. These findings indicate that the light peak represents a normal physiological interaction between the retina and the r.p.e. They also suggest that the interaction involves a change in the concentration of a diffusible substance in the retina, which then either enters the r.p.e. itself, or triggers an internal messenger to cause the basal depolarization.