Contrast sensitivity was measured for sinusoidal gratings sampled by compressing luminance into a variable number of sample bars. This procedure does not affect the amplitude or mean luminance of the grating, but does increase the local luminance of the sample bars: the fewer the bars, the more luminous they are. It was found that sensitivity increased with bars per cycle, particularly at low spatial frequencies. Further experiments, in which the local luminance of the sampling bars (but not the average luminance of the grating) was varied by addition of veiling glare showed that contrast sensitivity varied inversely with local bar luminance (a Weber type relationship). We interpret the results as evidence of local gain control under conditions where average luminance, and hence mean photon flux, does not vary. Calculations based on variation of sensitivity with spatial frequency suggest that gain control can be very localized, with receptive fields of Gaussian space constant of 0.5' arc. The relevance of these results to modern psychophysical concepts, including the definition of contrast is discussed.