Using an isometric strain gauge, we measured the twitch force produced in the first dorsal interosseous (FDI) muscle by a single anodal shock to the contralateral scalp. At high intensities of stimulation this twitch can greatly exceed the force produced by supramaximal stimulation of peripheral nerve. This indicates that a single cortical shock can cause repetitive firing of some or all FDI motoneurons. Such repetitive firing was demonstrated using a collision technique in which a supramaximal ulnar nerve stimulus was given at the wrist shortly after a cortical shock. The antidromic volley from the peripheral nerve stimulation failed to obliterate completely the response to cortical stimulation. Additional EMG activity was visible in the normally silent period between ulnar M and F waves. This activity must have been due to the presence of repetitive volleys of activity set up in spinal motoneurons by the cortical shock. Such activity summates with the twitch produced by the ulnar M wave to produce a very large force twitch of the muscle. Multiple firing of some motoneurons can be observed in some individuals at cortical stimulation intensities below that necessary to evoke activity in all the motoneurons in the FDI pool. Multiple firing probably was caused by repetitive excitatory inputs impinging on spinal motoneurons. These could be demonstrated using poststimulus time histogram techniques on single motor units. Low intensities of cortical stimulation produced a single short duration (mean 1.8 ms) peak of increased motor unit firing 20 to 30 ms after the shock. At high intensities, this was followed by extra peaks some 4 to 5 ms later. H reflex testing showed that the threshold of the initial descending volley, produced by the cortical stimulus was uninfluenced by a voluntary contraction. These results are discussed with reference to the D and I waves recorded from pyramidal tract after anodal stimulation of the exposed cortex in animals.