The receptive field organization of simple cells was studied by analyzing interaction effects between two stationary flashing light stimuli. One stimulus was positioned in the most responsive part of the receptive field to produce activity against which the effects of the other stimulus in various positions of the visual field could be determined. The receptive field was subdivided into an elongated center and elongated antagonistic flanks. The effects on the flanks were always considerably stronger on one side. Powerful flank suppression could be elicited within a region which usually was only slightly wider than the receptive field center. The suppression was just as stimulus specific as the activation of the center and occurred only by light ON or OFF. The cells were classified into ON-dominant and OFF-dominant depending on the kind of response found in the center. In ON-dominant cells the strong flank suppression occurred only by light ON, and light OFF produced enhancement. Correspondingly, the strong flank suppression occurred only by light OFF in OFF-dominant cells. This is consistent with the interpretation that simple cells have excitatory and inhibitory input from the same type of cells in the lateral geniculate nucleus (LGN), i.e., only from ON-center or OFF-center cells. The small size of the area where strong flank suppression occurred shows that inhibition comes fom a few LGN cells rather than from a large pool of cells. A model for simple cell receptive fields presuming overlapping but acentric excitatory and inhibitory fields with input to both fields frm either ON- or OFF-center LGN cells was tested by computer simulation and shown to fit the experimental data.