The middle ear is a gas pocket covered with a mucous membrane which contains a capillary network capable of gas exchange similar to lung alveolus, gastrointestinal mucosa and sinus mucosa. The pressure in the middle ear is the sum of the partial pressure of CO₂, O₂, N₂, and H₂O vapor. During general anesthesia, nitrous oxide enters the middle ear from the blood through the mucous membrane and a positive pressure develops in the middle ear. It is known that the mechanism for the increase in pressure is felt to be due to the difference in blood solubility between nitrous oxide and nitrogen. Nitrous oxide, being 34 times more soluble in blood than nitrogen, is carried to the air containing space in a greater quantity than the amount of nitrogen that can be carried away. In this study, middle ear pressure was obtained in 2 cases who were anesthized with nitrous oxide and in 9 cases who were anesthized without nitrous oxide using Danac 40 Impedance Audiometer. The results were as follows : 1) The middle ear pressure changed into positive pressure during general anesthesia with nitrous oxide. 2) The middle ear pressure changed into negative pressure during general anesthesia without nitrous oxide. 3) The ranges of minimum negative pressure were -138~-42 mmH₂O. 4) The times reaching to minimum negative pressure were 10~160 minutes. 5) After extubation in general anesthesia without nitrous oxide, the times reaching to normal pressure range were 10~130 minutes in room air. 6) Oxygen may be a cause decreasing the middle ear pressure.