Electromyographic (EMG) responses evoked in hand muscles by a magnetic test stimulus over the motor cortex can be suppressed if a conditioning stimulus is applied to the opposite hemisphere 6-30 ms earlier. In order to define the mechanism and the site of action of this inhibitory phenomenon, we recorded descending volleys produced by the test stimulus through high cervical, epidural electrodes implanted for pain relief in three conscious subjects. These could be compared with simultaneously recorded EMG responses in hand muscles. When the test stimulus was given on its own it evoked three waves of activity (I-waves) in the spinal cord, and a small EMG response in the hand. A prior conditioning stimulus to the other hemisphere suppressed the size of both the descending spinal cord volleys and the EMG responses evoked by the test stimulus when the interstimulus interval was greater than 6 ms. In the spinal recordings, the effect was most marked for the last I-wave (I3), whereas the second I2-wave was only slightly inhibited, and the first I-wave (I1) was not inhibited at all. We conclude that transcranial stimulation over the lateral part of the motor cortex of one hemisphere can suppress the excitability of the contralateral motor cortex.