Auditory evoked potential is an objective method for hearing test clinically and is also used for the diagnosis of pathologic condition in central auditory pathway. Among the auditory evoked response tests, middle latency response (MLR) has its latency from 10 to 50 msec. MLR is superior to other auditory evoked potentials in evaluating the hearing threshold at lower frequency, especially lower than 2KHz, and can be used as a supplementary test for auditory rehabilitation. The authors compared the MLR of non-sedation state(n=20) with that of sedation state(n=10) under the general anesthesia respectively. The frequency of applied stimuli was 1KHz and 2KHz each other. The intensity was set to 80dB in all situations. The latency of all waves revealed more prolongation in the stimulation with 1KHz compared to 2KHz one. In the both 1KHz and 2KHz stimuli, the latency was more prolonged in all waves on the sedation state under general anesthesia than in non-sedation state, but not significantly.