Responses of R/G-type horizontal cells recorded intracellularly in eye cup preparations with normal circulation of crucian carp under dark-adapted conditions were studied and photoreceptor inputs converging onto them were identified. Spectral sensitivities of the hyperpolarizing component of responses, determined at different phases, suggested that it was produced by input from green-sensitive cones. This input and input from long wavelength sensitive cones, causing depolarization of cells, were almost linearly summated. Based on the linear summation assumption, an algebraic procedure was developed to isolate the depolarizing component from responses of R/G-type cells to flashes at wavelengths longer than 540 nm. Spectral sensitivities of the isolated depolarizing component were determined and agreed reasonably well with those of red-sensitive cones. Therefore, it is unlikely that depolarization of R/G-type cells may be due to input from some presumptive deep-red sensitive cones.