The effects on median nerve somatosensory evoked potentials produced by nitrous-oxide (67%) were studied in 20 patients undergoing elective surgery under enflurane or isoflurane anaesthesia. This study demonstrates that nitrous-oxide does not significantly modify spinal (N13) and cortical (N20) component latencies nor central conduction time. The addition of nitrous-oxide to enflurane or isoflurane (up to 1 MAC) causes a significant reduction of N20 amplitude.