OBJECTIVE To determine the difference in performance of two different auditory evoked potentials (AEP) monitors, the A-Line AEP (AAI) and the aepEX, and their indices, during general anesthesia. METHODS Prospective study. METHODS Operating room at a private hospital. METHODS 40 ASA physical status I and II women, aged 30 to 70 years, scheduled for partial mastectomy. METHODS Anesthesia was induced with propofol and fentanyl, and a Laryngeal Mask Airway (LMA) was inserted. Anesthesia was maintained with propofol, fentanyl, and nitrous oxide. RESULTS The AAI or the aepEX was continuously monitored and their performance was compared at the start of monitoring, at LMA insertion, after disturbance by electric cautery, and during anesthesia. Eighteen of 20 patients had low enough impedance to extract good electroencephalogram signals at the first electrode application with the A-Line AEP, and 14 of 20 patients, with the aepEX. The time to return to good signals after signal disturbance by electric cautery was 14 +/- 3 seconds (SD) with the AAI and 19 +/- 4 seconds (SD) with the aepEX (P = 0.035). Both AAI and aepEX decreased after anesthesia induction, with significantly lower values seen in AAI than the aepEX. CONCLUSIONS The A-Line AEP (AAI) is better detects the response to painful stimuli and during recovering from noise of electric cautery than the aepEX. The aepEX shows higher values than the AAI during propofol-fentanyl-nitrous oxide anesthesia.