Neurons in the cat primary visual cortex are selective for particular contour orientations but their responsiveness can vary under certain conditions. After prolonged stimulation (adaptation), the contrast sensitivity of cortical cells is reduced and the 'gain' (the strength of response as a function of contrast) falls. The response to an optimal contour is also reduced when a different stimulus is superimposed on the receptive field in the same eye. Here we report that the sudden appearance of an inappropriate stimulus in one eye can interocularly suppress the activity of cortical neurons if they are already responding to an optimally oriented stimulus in the other eye. In strabismic cats, whose cortical neurons lack binocular facilitation, even contours of similar orientation shown to the two eyes trigger such suppression. This interocular control of cortical responsiveness could serve to veto signals from one eye under conditions that would otherwise cause double vision and perceptual confusion.