Lidocaine is most commonly used for local anesthesia in middle ear surgery. The purpose of this experiment was to investigate the effects of lidocaine on the cochlear microphonics and cochlear nerve compound action potential of guinea pig. The experiment was performed on 9 healthy guinea pigs, weighing 400-500gm. 0.1ml of 0.5% and 2.0% lidocaine and 2.0% lidocaine with 1 : 100,000 adrenaline was applied to the round window membrane of each 3 guinea pigs of each group. Cochlear microphonics (CM) and cochlear nerve compound action potential (AP) produced by multiple sound stimuli of 0.5, 1, 2, 4 and 6KHz were observed and recorded at 0, 30, 60 and 120 minutes after lidocaine application. The conclusions are as follows ; 1) Lidocaine increases CM amplitude and prolongs AP latency. 2) In the responses to the lidocaine of low concentration, both CM amplitude and AP latency recover until 2 hours after application of the drug to the round window niche. In the responses to the drug of high concentration, however, the both do not show significant recovery until 2 hours later. These mean that the effects of lidocaine on the CM amplitude and AP latency depend upon the concentration and the drug of high concentration may give some damage to the cochlea. 3) Adrenaline relieves much the effects of lidocaine on CM amplitude and AP latency. 4) The effects of lidocaine are more marked in the higher frequency zone (4 and 6 KHz).