The intracellular responses of frog horizontal cells and photoreceptors to conventional and randomly modulated stimuli were recorded by intracellular probing using glass micropipettes. The stimuli were designed to test for photoreceptor response compression by surround illumination. The results indicated that the response of photoreceptors is reduced by stimulation of the area surrounding the cell's receptive field. It appeared that the effect is generated by negative feedback from horizontal cells to both rods and cones. These findings are in good agreement with the results of earlier studies by other investigators who established that horizontal cells feed back to cone photoreceptors in many vertebrate species.