Mid-latency auditory evoked potentials (MLAEP) reflect the primary cortical processing of auditory stimuli. They are suppressed widely during general anesthesia. Under ketamine, in contrast, MLAEP seem to be preserved. Ketamine exists in two optical isomers, S (+) ketamine and R (-) ketamine, which differ in their pharmocodynamic properties. S (+) ketamine has a higher anesthetic-hypnotic and analgesic potency than R (-) ketamine or the racemic mixture of S (+) ketamine and R (-) ketamine. In a blinded, randomized evaluation we compared the effect of induction of general anesthesia with the more potent ketamine compound--S (+) ketamine--to induction with the racemic ketamine on MLAEP in 60 patients scheduled for minor gynecologic procedures. Anesthesia was induced with S (+) ketamine (1 mg/kg Group I, n = 30) or an equianesthetic dose of racemic ketamine (2 mg/ kg, Group II, n = 30). Auditory evoked potentials (AEP) were recorded before, during, and after induction of general anesthesia. Latencies of the peaks V, Na, Pa, Nb, and P1 and amplitudes Na/Pa, Pa/Nb, and Nb/Pl were measured. A fast-Fourier transform was used to calculate the power spectra of the AEP. The baseline MLAEP peaks of the awake patients were of normal amplitude and demonstrated a characteristic periodic wave form morphology. Power spectra indicated high energy in the 30-40 Hz frequency range. After induction of general anesthesia with S (+) ketamine or racemic ketamine, there was no increase in latencies of peaks V, Na, Pa, Nb, or P1. No decrease in amplitudes Na/Pa, Pa/Nb, or Nb/Pl could be observed. There was no significant change in the power spectra. MLAEP do not change in amplitude or latency during induction of general anesthesia with S (+) ketamine or racemic ketamine. Primary cortical processing of auditory stimuli seems to be preserved under S (+) ketamine and racemic ketamine.