The activity of neurones in cat striate cortex in response to laterally moving sinusoidal gratings has been examined. The effect of prolonged visual stimulation with a high contrast moving grating stimulus of optimal orientation and spatial frequency was investigated in terms of the changes wrought in the relationship between response amplitude and stimulus contrast. The adaptation which resulted was characterized by an elevation of contrast threshold by about 0.4 log units and a reduction in the steepness of the relation between response and contrast by about 0.36 log units. Both simple and complex cells exhibited this behaviour. The similarity of these values to the size of contrast threshold elevations demonstrated psychophysically suggests a possible physiological basis for the perceptual phenomenon.