WHEN performing myringoplasties under inhalation anesthesia administered by orotracheal intubation the surgeon often observes a bulging of the skin or vein graft. The bulging increases until it is so pronounced that air escapes from under the edge of the graft. The graft is seen to collapse, but soon the bulging will reappear. Quite obviously a positive pressure is generated in the middle ear, and it is likely to assume that this is caused by diffusion into the middle ear of the anesthetic gas in use. Some preliminary experiments corroborated this assumption and indicated that the positive middle ear pressure had to be attributed to the use of nitrous oxide, as the bulging ceased shortly after discontinuing the supply of this gas and started again when the supply was resumed. This observation is in accordance with the knowledge of the middle ear pressure and its relation to the tensions of the