Preoperative recording of evoked potentials after stimulation of the tibial nerve was followed by similar intraoperative recording in 6 patients with cauda and conus terminalis tumours and 2 patients with intervertebral disc prolapse at lumbar level. The shape and latency of these potentials were sensitive indicators of the level and degree of damage to the nervous elements in this area. The selectivity of the method was significantly higher during intraoperative recording. Monitoring of these potentials may serve for control of the integrity of sensory pathways during neurosurgical interventions.