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Midlatency auditory evoked potentials (MLAEP) show graded changes with increasing doses of hypnotics but little change with opioids. The effect of their combination on the MLAEP was evaluated. Also, the bispectral index (BIS) was compared with the ability of MLAEP to correlate with sedation and predict loss of consciousness.Twenty healthy volunteers were randomly assigned to receive stepped increases in propofol concentration (10 subjects) or propofol plus alfentanil 100 ng/ml (10 subjects). At baseline and at each targeted effect site concentration the mean MLAEP, BIS, measures of sedation, and drug concentration were obtained. The relation among MLAEP, BIS, and sedation score was determined. The prediction probability (Pk) was calculated and compared for BIS and MLAEP.The BIS and MLAEP patterns showed significant changes (Pa and Nb decreased in amplitude and increased in latency) with increasing level of sedation (P < 0.0001). The BIS correlated better with sedation scores (0.884) than did the MLAEP (P < 0.05). Pa and Nb latencies showed the best correlation with sedation levels (0.685 and 0.658, respectively). The addition of alfentanil did not affect the relation between MLAEP and loss of consciousness (P > 0.15). The BIS (Pk = 0.952) was a better predictor of loss of consciousness than were Pa and Nb amplitude (P < 0.05) but were comparable to Pa and Nb latency (Pk = 0.869 and 0.873, respectively).MLAEP changes, like the BIS, correlate well with increasing sedation produced by propofol, and these changes in the MLAEP are independent of the presence of an opioid. Among all the MLAEP parameters, Pa and Nb latencies are the best predictors of increasing sedation and loss of consciousness.