We have studied differential effects of propofol and nitrous oxide on posterior tibial nerve somatosensory evoked potentials (PTN-SEP) during a continuous infusion of alfentanil. In study 1, 14 patients received initially 66% nitrous oxide in oxygen, which was replaced 90 min after incision by propofol 6 mg kg−1 h−1. This substitution resulted in a significant increase in mean P1N1 amplitude from 1.01 (SD 1.14) μV to 2.61 (2.17) μV (P < 0.01), while amplitudes of later peaks were unaffected. Latencies of peaks N1, P2 and N2 increased after the substitution. In study 2, 30 patients undergoing spinal surgery received either alfentanil-nitrous oxide anaesthesia (group l, n = 15) or alfentanil-propofol anaesthesia (group ll, n = 15). P1N1 amplitude was significantly greater in group ll (3.24 (1.08) μV) than in group l (1.64 (0.97) μV) (P < 0.01). Latencies of peaks P2 and N2 were of significantly greater duration in group II than in group I. Because early cortical PTN-SEP peaks were preserved better during alfentanil-propofol anaesthesia we conclude that this combination may be a suitable alternative to alfentanil-nitrous oxide anaesthesia, when spinal cord function monitoring with PTN-SEP is indicated.