Contrast increment thresholds were measured for light and dark bars as a function of the base contrast of the bars. The bars were superimposed on a uniform field of 340 cd/m2. They had either rectangular or Gaussian luminance profiles, varied in width from 0.1 degree to 10 degrees, and in duration from 10 to 200 msec. For the 200-msec presentations, the resulting contrast discrimination functions all had approximately the same shape when contrast was defined as (Lmax - Lmin)/(Lmax + Lmin), and closely resembled corresponding results for sine-wave gratings. The similarity in shape of contrast discrimination functions for light and dark bars is attributed to a retinal nonlinear intensity transformation. The 10 msec contrast discrimination functions differed from the 200-msec functions in ways that can be explained by differences in temporal integration.