Changes in Sensory Evoked Potentials (SEPs) produced by different concentrations of isoflurane with or without N2O have been studied. In all subjects the early components have always remained well defined. A significant increase of N 13 and N 20 latency and of the central conduction time has been noted. The amplitude of N 20 was not much modified by isoflurane, but was affected by the addition of N2O. Isoflurane may be used during SEP monitoring if its concentration remains below 1 MAC.