Motor evoked potentials (MEPs) induced by transcranial magnetic stimulation were investigated as a means of assessing motor function in mice subjected to spinal cord crush injury. Compression injuries were produced by crushing the spinal cord at T8 using an extradural approach. MEPs as well as Tarlov scores were recorded prior to injury and then weekly for 4 weeks (6 months in some animals) after injury. Immediately after the surgery, the animals exhibited complete hindlimb paralysis and loss of bladder function. Before the crush, MEPs recorded from hindlimb muscles following transcranial magnetic stimulation consisted of a relatively stable early biphasic response (2.5 [plusmn] 0.85 ms) and a later low-voltage, multiphasic response (6.5 [plusmn] 2.05 ms). Immediately after the crush, the early component of the MEP of every animal was markedly decreased or absent and the Tarlov score was 0 [plusmn] 0. The early component returned to approximately 16.7% of pre-crush baseline amplitude within 3-4 weeks, at which time, the mean Tarlov score was 2.27. The late multiphasic components never recovered at any time point.Histopathology 3-4 weeks after crush revealed no intact axons in the ascending proprioceptive tract or in the descending corticospinal tract which in the mouse, is also in the dorsal column. The results suggest that experimental spinal cord crush injury in mice produces a significant abnormality in MEPs immediately after the crush, and gradual recovery which correlates with recovery of hindlimb movements. This hindlimb function presumably reflects sparing of ventrally located descending tracts. These observations indicate that MEPs by transcranial magnetic stimulation can be useful in the quantitative evaluation of the motor function after spinal cord injury in mice.