The ventilation mechanism of the middle ear is very important as regards the pathogenesis of middle ear disease, but its mode of function is still obscure. Therefore, we tried to measure the ventilation and clearance of the middle ear using radio-isotope imaging techniques and 133Xe in order to gain a clearer picture of the ventilation mechanism. In normal Eustachian tube cases, approximately 10% of the initially insufflated gas immediately entered into the middle ear cavity and mastoid air cells. The gas introduced into the middle ear diminished at a rate of 8% of volume per hour in the normal resting state. Two hours after the first procedure, air was insufflated, and gas volume in the middle ear cavity immediately diminished by 30%. In stenotic tube cases, it proved difficult to insufflate the gas into the middle ear, however, its diminishing rate with the passage of time was slightly faster than in normal tube cases. From this data, it was evident that air could easily and quickly enter into even the periphery of the mastoid air cells by insufflation via the Eustachian tube, despite the fact that the middle ear and mastoid air cells form a closed cavity. In the resting state, moreover, the air in the middle ear was thought to be absorbed mainly into the middle ear mucosa at a regular rate. It was confirmed that the insufflation procedure as a therapy for tubal stenosis and OME is very useful for the ventilation of the middle ear.