1. Observations were made on seven human subjects with electrodes implanted in the cerebellar thalamus for the control of tremor. 2. Weak stimulation at 1-2 Hz resulted in sudden lapses of posture of actively contracting contralateral limb muscles. Stronger stimuli caused muscle twitches even in relaxed muscles. 3. The stronger stimuli produced short latency facilitation of EMG activity in contralateral muscles; the pattern of muscle facilitation, the conduction velocity of the descending pathway and the estimated rise time of the underlying composite EPSP were consistent with direct activation of the corticospinal tract. 4. The lapses of posture produced by the weaker stimuli were associated with inhibition of ongoing EMG for up to 150 ms. This was due to the interruption of tonic drive to motoneurons rather than to their inhibition and was associated with depression of the motor evoked potential in relaxed muscles produced by weak anodal transcranial stimulation. The inhibition could be produced by 0.1 ms pulses, implying that a large-diameter fibre system was being activated. 5. A number of mechanisms could contribute to the inhibition, including inhibition from the reticular nucleus of the thalamus.