Extracellular microelectrode recordings were made from ganglion cells of the intact, in situ eyes of adult common domestic cats. Three different photopic systems, with peak spectral sensitivities at 450, 500, and 556 nm, were observed. All ganglion cells received input from a cone system with a peak spectral sensitivity of 556 nm. The blue-sensitive cone system was observed in about one-half of the ganglion cells studied. In each case the 450-nm cone system contributed to only one functional type of response, either ON or OFF, in the same cell. The other two photopic systems most often contributed to both the ON and OFF responses of an individual ganglion cell. In four cases the 450-nm cone system mediated responses that were opponent to those of the other two photopic systems. The third photopic mechanism has a peak spectral sensitivity at 500 nm and contributed to most receptive field surrounds and many receptive field centers. It is distinguished from the rod system by the occurrence of a break in both dark-adaptation curves and increment-sensitivity curves. No apparent differences in receptive field cone contributions between brisk-sustained and brisk-transient cells were seen.