Strabismus and anisometropia are generally accompanied by a sensory adaptation in the form of some suppression of the input from the affected eye. In this study identical images of less than 0.5º of visual angle were presented to the two eyes of normal subjects by means of a synoptophore and their positions adjusted until they became fused. A reversing checkerboard stimulus was added to the background field of either eye to obtain a monocular visual evoked response (VER) during binocular viewing of the effused synoptophore image. Defocusing of the synoptophore image to one eye by approximately 2 dioptres reduced (to 50%) the amplitude of the VER from the same eye. Occlusion of the unstimulated eye greatly increased response amplitude and eliminated the effect of defocusing. Most subjects were able to maintain a subjectively clear synoptophore image though the image to one eye was defocussed. The study confirms that under binocular viewing conditions normal subjects can effectively suppress an inferior input to one eye. It is proposed to use a similar technique to explore the suppressing abilities of strabismic and anisometropic subjects.