We compared the effects of morphine and fentanyl in the presence of 60% N2O on somatosensory cortical-evoked potentials (SCEP). Both drugs were administered by intravenous bolus (n = 12) and infusion (n = 20) techniques. SCEPs were recorded preoperatively and intraoperatively in 32 patients undergoing corrective surgery for scoliosis. Records were taken at the contralateral cerebral cortex by individual stimulation of the posterior tibial nerves at the ankle. Both drugs increased the latencies of the N1, P2, and N2 peaks and affected the peak-to-peak amplitudes of the primary complex. Intravenous bolus injections and continuous infusions of equianalgesic doses produced similar effects. The increase in N1 latency was significantly greater (morphine, P less than 0.05; fentanyl, P less than 0.01) with the bolus than with the infusion technique. The doses of morphine and fentanyl given by bolus injections were 1/3 times and 3 1/3 times greater than doses given by infusion. Changes in latency were more consistent than changes in amplitude. Both fentanyl and morphine increase latencies while affecting amplitudes unpredictably. Equianalgesic doses of fentanyl and morphine have similar effects on SCEP latencies. Low-dose continuous infusions of narcotics depress SCEPs less than intermittent bolus injections.