Compound muscle action potentials (CMAPs) evoked by magnetic brain stimuli are larger if the subject provides a steady background voluntary contraction of the target muscle. This facilitation could be due either to cortical or spinal mechanisms, or both. Both magnetic and electrical stimuli given immediately after the onset of a ballistic contraction also evoke markedly facilitated CMAPs. By contrast, responses some 200 ms after the onset of such a contraction are facilitated if stimuli are magnetic but not if they are electrical. This second phase of facilitation is largely cortical in origin. By comparing the size of CMAPs evoked by magnetic stimuli at two different delays after electromyogram onset, the total facilitation could be dissected into its spinal and cortical components. The relationship between CMAP area in the first dorsal interosseous and stimulus intensity was different in the two phases of facilitation, suggesting a constant background level of spinal facilitation upon which an increasing descending volley operated. In experiments in which ballistic contractions at increasing force levels were performed it was found that at low force levels, spinal facilitation predominated, but at forces greater than 10% maximum there were roughly equal contributions from increased spinal cord and cortex excitability.