We recorded visual evoked potentials in response to square-wave contrast-reversal checkerboards undergoing a transition in the mean contrast level. Checkerboards were modulated at 4.22 Hz (8.45 Hz reversal rate). After each set of 16 cycles of reversals, stimulus contrast abruptly switched between a "high" contrast level (0.06 to 1.0) to a "low" contrast level (0.03 to 0.5). Higher contrasts attenuated responses to lower contrasts by up to a factor of two during the period immediately following the contrast change. Contrast-response functions derived from the initial second following a conditioning contrast shifted by a factor of two to four along the contrast axis. For low-contrast stimuli, response phase was an advancing function of the contrast level in the immediately preceding second. For high-contrast stimuli, response phase was independent of the prior contrast history. Steady stimulation for periods as long as one minute produced only minor effects on response amplitude, and no detectable effects on response phase. These observations delineate the dynamics of a contrast gain control in human vision.