The responsivity of primary sensory cortical neurons is reduced following prolonged adaptation, but such adaptation has been little studied in higher sensory areas. Adaptation to visual motion has strong perceptual effects, so we studied the effect of prolonged stimulation on neuronal responsivity in the macaque's area MT, a cortical area whose importance to visual motion perception is well established. We adapted MT neurons with sinusoidal gratings drifting in the preferred or null direction. Preferred adaptation reduced the responsiveness of MT cells, primarily by changing their contrast gain, and this effect was spatially specific within the receptive field. Null adaptation reduced the ability of null gratings to inhibit the response to a simultaneously presented preferred stimulus. While both preferred and null adaptation alter MT responses, these effects probably do not occur in MT neurons but are likely to reflect adaptation-induced changes in contrast gain earlier in the visual pathway.