The organization of geniculate inputs to a cat's visual cortical cell was studied by a cross-correlation technique. Simultaneous extracellular recordings were made in the lateral geniculate nucleus and in the striate cortex, and neuronal connectivity between a geniculate cell and a striate cell was examined by cross-correlograms of their impulse discharges under photic stimuli. Of 243 pairs of geniculate and striate cells with overlapping receptive fields, 82 showed positive correlations with short (0.9-2.7 ms) delay times. The delays in 65 of the 82 pairs were short enough to infer that the geniculate cell exerted monosynaptic excitatory action on the striate cell. Monosynaptic excitations were found in all types of striate cells. Those in cells with exclusively an on area or an off area (E-on/off cells) or in simple cells originated mostly from X geniculate cells; those in special-complex cells originated exclusively from Y geniculate cells; and those in standard-complex cells arose from both X and Y geniculate cells. The convergence number from geniculate cells to an E-on/off or simple striate cell was estimated as more than 10, since about 1/10 of the discharges from an E-on/off or simple cell in response to a moving stimulus was correlated with discharges from a geniculate cell. A larger convergence number (more than 30) was obtained for complex cells. Convergence from 2 to 5 geniculate cells was actually demonstrated in 17 of the 32 striate cells, each of which was tested in pair with 3-14 geniculate cells. The converging inputs thus observed included both X and Y geniculate cells in one E-on, one simple, and three standard-complex cells. They included both on-center and off-center geniculate cells in one simple, one special-complex, and five standard-complex cells. Under stimulation with a stationary light slit, the center fields but not the surround fields of geniculate cells were found to contribute to the receptive fields of the simple striate cells. However, the surround fields of geniculate cells contributed to the subliminal response areas flanking the central areas of E-on/off cells. The center fields of the geniculate cells also contributed to the central areas of the E-on/off cells. These observations suggest different models for simple cells and E-on/off cells as regards the organization of their geniculate inputs; simple cells may receive inputs from both on-center and off-center geniculate cells, but E-on/off cells receive inputs only from one or the other of them.