In 30 patients undergoing spinal disc operations, the effects of bolus injections followed by intravenous infusions of thiopental, etomidate, and midazolam on median nerve somatosensory-evoked potentials (SSEPs) were studied. Possible additive effects of fentanyl and nitrous oxide were also evaluated. Serial SSEP measurements were made before and for 25 minutes after the start of anesthesia. After induction with one of the three intravenous agents, fentanyl (10 micrograms/kg) was administered and SSEPs were again measured 1 and 5 minutes after administration. Sixty-five% nitrous oxide in 35% oxygen was administered after tracheal intubation and was followed by final SSEP measurements. The three intravenous agents affected SSEP signals differently. Etomidate increased both amplitude and latency. Thiopental decreased amplitude and increased latency. Midazolam had no effect on amplitude but increased latency. The addition of fentanyl and nitrous oxide had different effects in response to the three intravenous induction agents. This study emphasizes the differences in SSEP responses not only to different intravenous induction agents but also to the addition of fentanyl and nitrous oxide.