This study was designed to determine whether alterations in the median nerve somatosensory evoked potentials occur during the stimuli of tracheal intubation and skin incision. Twenty-two patients scheduled for elective surgery and who required tracheal intubation were studied. Median nerve somatosensory evoked potentials were recorded, analysed and stored approximately every 40 seconds. Anaesthesia was induced with thiopentone and vecuronium used for neuromuscular blockade; the trachea was intubated 2 minutes after induction. Fentanyl 1.5 micrograms/kg was administered subsequently. Evoked potential monitoring was continued until at least 2 minutes after surgical incision. Induction of anaesthesia was associated with an increase in evoked potential latency of 0.8 msec and reduction in amplitude of 1.7 microV. Small, statistically insignificant changes occurred between induction of anaesthesia and tracheal intubation. Surgical incision was accompanied by a statistically significant mean decrease in evoked potential latency of 0.5 msec and a statistically significant increase in evoked potential amplitude of 0.6 microV. The fact that surgical stimulation produced an activating effect on evoked potentials suggests that they may be used as a measure of the neurophysiological effects of anaesthesia.