In healthy human subjects, descending motor pathways including the corticospinal tract were stimulated electrically at the level of the cervicomedullary junction to determine the effects on the discharge of motoneurones innervating the biceps brachii. Post-stimulus time histograms (PSTHs) were constructed for 15 single motor units following electrical stimulation of the corticospinal tract and for 11 units following electrical stimulation of large diameter afferents at the brachial plexus. Responses were assessed during weak voluntary contraction. Both types of stimulation produced a single peak at short latency in the PSTH (mean 8.5 and 8.7 ms, respectively) and of short duration (< 1.4 ms). In separate studies, we compared the latency of the responses to electrical stimulation of the corticospinal tract in the relaxed muscle with that in the contracting muscle. The latency was the same in the two conditions when the intensity of the stimulation was adjusted so that responses of the same size could be compared. Estimates of the descending conduction velocity and measurements of presumed peripheral conduction time suggest that there is less than 0.5 ms for spinal events (including synaptic delays). We propose that in response to electrical stimulation of the descending tract fibres, biceps motoneurones receive a large excitatory input with minimal dispersion and it presumably contains a dominant monosynaptic component.