Background: Transcranial electrical motor-evoked potentials (TceMEPs) can provide early warning of possible motor compromise during surgery. There are fewer reports comparing the effects of etomidate and propofol infusion on TceMEPs when used for the maintenance of anesthesia and guided by comparable values of bispectral index (BIS) during spinal surgery. Methods: Thirty-three patients scheduled for spinal surgery were randomly divided into 2 groups: propofol (PR, n = 18) and etomidate (ER, n = 15). Anesthesia was maintained with either propofol or etomidate combined with remifentanil. The infusion rates for propofol or etomidate were guided by the BIS value, which was maintained between 40 and 45. TceMEPs were conducted by stimulating needles placed at C1 and C2; recordings were made by measuring myogenic responses from the upper extremity abductor pollicis brevis muscles using needle electrodes. The threshold for eliciting a response, amplitudes, and latencies of TceMEPs, were recorded at 30, 60, 90, and 120 minutes after the induction of anesthesia. The cortisol levels were measured at 2 and 24 hours after induction. Results: The voltage threshold needed to enlist TceMEPs in the ER group was significantly lower than that in the PR group (142 +/- 20 vs. 172 +/- 23 V, P = 0.005). The amplitudes of TceMEPs were higher in the ER group than those in the PR group (P < 0.05), whereas the latencies were shorter in the ER group than those in the PR group (P < 0.05) at all study time points. Cortisol levels at all study time points were within the normal range. Conclusions: Etomidate has more favorable effects than propofol during the monitoring of TceMEPs under comparable BIS levels.