Intracellular recordings of horizontal cells were made in isolated carp (Cyprinus carpio) retina. The area-response amplitude relation and the potential decay with distance were explored on each component of S-potentials induced by appropriate monochromatic lights. The response amplitude was increased approximately linearly as the spot diameter was increased from 0·5 to 5·1 mm, except for the scotopic L-type response, which was increased exponentially and showed a narrower area of potential summation than those of other type responses. The potential decay curve for each component of S-potentials, explored with displacement of a 0·5 mm light spot in a range of 5 mm length over the retinal surface, was found to vary with different type cells and different components of a given C-type cell. The potential decay of the hyperpolarizing responses of scotopic and photopic L-type cells, and of RG- and YRB-type cells (at shorter wavelength) formed a unimodal curve; the maximum amplitude response was obtained at the recording point. On the other hand, the potential decay of the depolarizing and second hyperpolarizing response of C-type cells drew a bimodal curve, in which the amplitude of response at the recording point was slightly smaller than those obtained at some distances on the bilateral sides. The results indicate that the different components of S-potentials spread laterally in different modes along even a given C-type cell layer. The phenomena observed were interpreted on the basis of postulated interconnections between cones and horizontal cells.