The middle ear is a cavity surrounded by solid bones, lined with mucosa, which has a gas-filled lumen. Cavernous organs such as the ear should have their own ventilation system under atmospheric pressure. The mechanism of ventilation in the middle ear has not been sufficiently clarified. Ventilation performed in the middle ear may be classified into two types: 1) passive ventilation via the Eustachian tube, required in cases of abnormal pressure and 2) unique physiological active ventilation of the middle ear performed under atmospheric pressure and not involving the Eustachian tube. The purpose of the present study is to prove the existence of this active ventilation under atmospheric pressure. The subjects were 50 normal ears and elevation of middle ear pressure in the lateral position (determined by positional tympanometry) was studied. The change in the peak level, on tympanometry, was used as an index. The results were continuously recorded every 12 seconds. The following results were obtained. 1. Middle ear pressure was elevated by changing from the sitting to the lateral position. Venous pressure was regarded as a causative factor in this pressure elevation. 2. The elevated middle ear pressure in the lateral position suggested gas production from mastoid cells of the middle ear. The observation that the middle ear pressure was stabilized with the increase in pressure, up to a level of 85-90 mm H2O, indicated the existence of gas leakage from the Eustachian tube and a mechanism for controlling gas production from the mastoid cells of the middle ear.