The eye functions effectively over an enormous range of ambient illumination, because retinal sensitivity can be adapted to prevailing light levels. Higher order neurones in the visual pathway are presumably more concerned with relative changes in illumination, that is, contrast, because a great deal of information concerning absolute light level is processed at the retinal level. It would therefore be of considerable functional value if cells in the visual cortex could adapt their response levels to a steady-state ambient contrast, in a manner analogous to the sensitivity control mechanism of the retina. We have examined here the idea that adaptation of neurones in the visual cortex to ambient contrast is similar to adaptation in the retina to ambient illumination. The experiments were performed by measuring contrast response functions (response amplitude as a function of contrast) of striate neurones, while systematically adapting them to different contrast levels. Our results show that, for the majority of cortical neurones, response-contrast curves are laterally shifted along a log-contrast axis so that the effective domains of neurones are adjusted to match prevailing contrast levels. This contrast gain control mechanism, which was not observed for lateral geniculate (LGN) fibres, must be of prime importance to visual function.