To investigate the sites of light adaptation processes in the mammalian distal retina, we studied the lateral spread of adaptation signals in cone-driven cat horizontal (H-) cell responses. The size of the adaptation pool is compared to the receptive field for H-cell responses. H-cell activity was recorded intracellularly in the optically intact, in vivo eye. It is demonstrated that light adaptation as measured in H-cells is not a strictly local process. Background light falling outside a central test region effectively modulates the responses to a small test light, flashed on the receptive field center. The integration area for adaptation signals was quantitatively compared to the H-cell receptive field size by measuring the desensitizing effect of background light on the responses to a small centered test spot, as a function of background spot size. The area-adaptation function is comparable to the area-response function but has a slightly smaller length constant. Light adaptation in H-cell responses, therefore, reveals spread of adaptation over a large distance and is probably mediated through lateral interactions in the H-cell network rather than in the cones.