The evoked compound action potential of lumbar nerve roots after posterior tibial nerve stimulation at the ankle during operation were studied in 10 patients with lumbar disc herniation. The recording needle electrode was inserted into compressed the nerve root before and after excision of disc herniation. The evoked nerve root action potentials consisted of a positive-negative complex. Before excision, this predominant positive complex was recorded in every patient with a mean peak latency of P1 at 16.64 +/- 1.69 msec, and a mean peak to peak amplitude of P1-N1 of 1.94 +/- 0.52 microv. In three cases with migratory disc herniation, the P1-N1 complex was followed by smaller negative component. After excision of disc herniation, the amplitude of the P1-N1 complex decreased significantly with no significant latency modification. The second negative component was not recorded in the three cases with migratory disc herniation. The recording of nerve root evoked potential during operation is of interest for diagnostic purposes. In addition, it allows electrophysiological monitoring of nerve root traumatism during operation procedures.