We examined the spatiotemporal organization of excitatory regions in 197 simple receptive fields from cat area 17 using the peristimulus time response-plane technique of Stevens and Gerstein (53). With this method we observed a striking similarity between the spatiotemporal organization of excitatory regions in simple receptive fields and the excitatory centers in X or Y geniculate receptive fields. This observation suggested to us the possibility that individual simple receptive fields may be differentially innervated by either X or Y geniculate afferents. To test this hypothesis, we devised a quantitative measure that could characterize the excitatory regions in simple receptive fields as being X-like or Y-like. This measure was based on an understanding of the spatiotemporal organization of geniculate X and Y receptive fields. Further evidence supporting this division of simple cells was derived from additional physiological and anatomical comparisons. When compared to Y-like simple cells, X-like simple cells, as a group, gave a more sustained response to standing contrast, had smaller excitatory regions, and preferred slightly slower moving stimuli. A comparison of the properties of end-zone inhibition and directional selectivity showed no additional difference between X-like and Y-like simple cells. We found a correlation between the laminar position of X-like and Y-like simple cells and the known patterns of termination of X and Y geniculate afferents. Y-like simple cells were found in layers III, IVab, and VI, but not in layer IVc, whereas X-like simple cells were found in layer III, all parts of layer IV, and layer VI. Inhibitory regions appeared to play a major role in defining the spatiotemporal structure of simple receptive fields and they further acted to diminish differences between the spatial widths and velocity sensitivities of X-like and Y-like simple cells. These data are discussed in terms of a parallel model of geniculostriate convergence and support the hypothesis that the X and Y systems, which originate in the retina, are maintained in parallel at the level of simple cells in striate cortex.