Sensitivity to motion was measured by the percentage of trials on which an observer reported seeing motion of briefly presented high-contrast sinusoidal gratings moving over a range of velocities. The psychometric curve was remeasured following adaptation to a grating moving in one direction for an extended period of time. Adaptation shifted the minimum of the psychometric curve toward the direction of the direction of the adapting stimulus. The shift was smaller when the adapting field was larger than the test. In a second set of experiments we measured the effect of motion adaptation on contrast thresholds for moving gratings of different sizes. Threshold elevation was maximal when adapting and test sizes matched. We present a mechanistic model of the motion aftereffect that consists of independent multiplicative gain controls in motion-sensing mechanisms tuned to different rates of motion. In addition, we discuss a model of size effects in motion adaptation that invokes diffuse inhibitory connections among motion-sensing mechanisms.