N2O can diffuse in and/or out the middle ear cavities and it may alter the middle ear pres-sure. This study was performed to investigate the effect of N2O discontinuation on the changes in middle ear pressure during general endotracheal anesthesia. We measured middle ear pres-sures and end-tidal NO concentrations at preanesthesia, immediately after N2O discontinuation and then at every five minutes for 60 minutes under either halothane or enflurane an- esthesia in two groups(group 1=50% administration of N2O in O2; group 2=60% administration of N2O in O2). The middle ear pressures after N2O discontinuation decreased slowly but they did not returned to the preanesthetic value in both groups. The end-tidal N2O concentrations decreased rapidly until five minutes and then they decresed slowly to the zero leve1 until 30 minutes after N2O discontinuation in both groups. The expected time of the middle ear pres-sure of preanesthesia value was 89.4 minutes in group 1 and 80.9 minutes in group 2 respectively. The middle ear pressure do not return to the level of preanesthesia at 60 minutes after N2O discontinuation while the end-tidal N2O concentration decrease to the zero level via 30 minutes after its discontinuation. Thus it is recommended that N2O administration should be discontinued at least 30 minutes before an application of tympanic membrane patch on the ear drum and N2O is carefully administered in patients with middle ear and/or upper airway disesses.