The time course of facilitation of motor evoked potentials (MEPs) to transcranial electrical stimulation delivered at varying intervals near the onset of a voluntary ballistic movement was studied in 4 normal subjects. MEPs were recorded from the left thenar muscles to unifocal anodal stimulation of the right scalp overlying the hand motor area delivered every 8-10 sec. A click, occasionally associated with the scalp stimulation (P = 0.3-0.6), was the signal for the subject to make a brief thumb press on a piston at short latency. The timing of the scalp stimulus and the click was adjusted so that the former occurred approximately between 100 msec before and 100 msec after the onset of the voluntary movement signaled by the EMG in the thenar muscles. MEPs were not detected when the scalp was stimulated 80 msec or more before onset of voluntary movement and then appeared with increasing probability as the time interval before movement shortened. The amplitudes of MEPs in the 80-40 msec period preceding movement onset were small (less than 20% of maximum) and achieved maximum values 20 msec after movement onset.