The response to partial denervation is compared for the tibialis anterior muscle in the rat and the thenar muscle group in the human. Partial denervation in the human was a result of spinal cord injury, while partial denervation in the rat was induced by sectioning of the L4 ventral root. In some animals, a spinal cord transection at the T12-13 level was also performed to determine whether spinal cord injury affected the sprouting capability of motoneurons caudal to the injury site. Motor units were isolated by intramuscular microstimulation in the human and by ventral root splitting in a terminal experiment in the rat. Motor unit numbers were estimated by dividing the amplitude of the electromyogram (EMG) and the peak twitch force in response to maximum stimulation of the muscle nerve by the average unit EMG and twitch force, respectively. In both the rat and the human, surviving motor units enlarged as a function of the degree of partial denervation. Moreover, all surviving motor units appeared to enlarge proportionately. The limit to sprouting was tested systematically in the rat. On average, single motor units enlarged up to about five times their original size, resulting in the ability to compensate for up to 80% of motoneuron loss. The reason for this limit remains unclear, but histological data suggest that sprouting may be confined to the more distal regions of the motor axon, such that reinnervation of denervated muscle fibers is confined to the territory of the original motor unit.