During adaptation to a high contrast grating it gradually seems to fade. A lower-contrast test grating appearing directly after the adapting pattern appears reduced in apparent contrast. The orientation specificity and spatial frequency specificity of thisapparent contrast reduction have been determined by adapting to gratings of various orientations and spatial frequencies, and measuring the contrast reduction for test gratings of fixed orientation and frequency. The sensitivity characteristic for orientation has a half-width at half amplitude of 8°: that for spatial frequency has a full-width at half amplitude of 0.75 octaves. This result is compared with the properties of neurones in the cat and monkey visual cortex.