A need exists for an accurate neurophysiological technique that monitors the motor tracts of the cord in patients with spinal cord injury or other cord lesions and for the evaluation of experimental models of cord injury. We have recorded and characterized the motor evoked potentials (MEPs) from 10 normal rats and from 10 rats with the following cord lesions at C-8: 4 animals with complete cord transection and 6 with clip compression injury: 2 at 56.0 g, 2 at 20.0 g, and 2 at 1.5 g. MEPs were elicited by applying constant current anodal stimuli to the sensorimotor cortex with the responses recorded from microelectrodes in the spinal cord at T-10. The MEP consisted of an initial D wave (mean latency, 1.22 +/- 0.09 ms) and 4 subsequent I waves, I1 to I4. The D wave was elicited at stimulation frequencies exceeding 100 Hz, consistent with the hypothesis that it results from direct pyramidal cell excitation. The 56.0-g clip compression injuries and the cord transections abolished the MEP distal to the lesion, whereas the 20.0- and 1.5-g injuries resulted in a latency shift and amplitude decrement of the MEP peaks. These experiments suggest that the recording of MEPs will be an extremely useful and accurate method of monitoring the functional integrity of the cord, of value in patients with cord injury.