The somatosensory evoked potential in response to median nerve stimulation was recorded in 42 patients during infusion of either 15 mg/kgbw thiopentone (TH) or 1 mg/kgbw etomidate (E) within 15 min and before and after injection of 0.3 mg/kgbw etomidate bolus. Cortical and cervical responses were analysed simultaneously and central conduction time (CCT) was calculated. Marked alterations of waveforms and an increase in latency of the primary cortical SEP and of CCT were observed in all patients. Infusion of TH or E was followed by a diminution of middle and long latency components. Amplitude of the cortical N20 was found to be unchanged during and after TH and to be increased after infusion or injection of E, indicating the synchronizing properties of this drug. The cervical SEP (N14) remained entirely unchanged in response to both agents. During hypnotic drug administration a pronounced increase in latencies and CCT as well as a decrease in the number of identifiable peaks has to be considered when SEP monitoring is performed intraoperatively or in intensive care treatment.