We have studied the effects of voluntary attention on the induction of motion aftereffects (MAEs). While adapting, observers paid attention to one of two transparently displayed random dot patterns, moving concurrently in opposite directions. Selective attention was found to modulate the susceptibility to motion adaptation very substantially. To measure the strength of the induced MAEs we modulated the signal-to-noise ratio of a real motion signal in a random dot pattern that was used to balance the aftereffect. Results obtained for adapting to single motion vectors show that the MAE can be represented as a shift of the psychometric function for motion direction discrimination. Selective attention to the different components of transparent motion altered the susceptibility to adaptation. Shifting attention from one component to the other caused a large shift of the psychometric curves, about 70-75% of the shift measured for the separate components of the transparent adapting stimulus. We conclude that attention can differentiate between spatially superimposed motion vectors and that attention modulates the activity of motion mechanisms before or at the level where adaptation gives rise to MAEs. The results are discussed in light of the role of attention in visual perception and the physiological site for attentional modulation of MAEs.