Several physiological mechanisms contribute to the shaping of the pressure variations in the middle ear. The most important of these are resorption of gas from the middle ear, the function of the eustachian tube, the mechanical properties of the ear drum system, the volume of gas filled ear space and the volume changes of the mucosa of the middle ear. Gas exchange in the middle ear cavity potentially occurs in three ways ; by diffusion across the tympanic membrane, by the venting action of the eustachian tube, by diffusion through the capillaries of the mucosal layer of the middle ear and the bulla. Most research on the gas turnover has been based on the presumption that oxygen is continuously being absorbed through the mucosal lining of the middle ear cavity. But, recent studies suggest that PCO₂ of the blood may affect the middle ear pressure(MEP). In order to study the middle ear pressure variations due to PCO₂ changes of the blood, the MEP was measured by impedance audiometer(GSI 28 Auto Tymp model) during general anesthesia from Nov. 1985 to Mar. 1986 at Kangnam sacred heart hospital. The first group consisted of five persons with healthy ears and otoscopically normal tympanic membrane, and normal, ""type A"" tympanograms. The patient was anesthesized with pentothal sodium and maintained with 1-2 volume % halothane and 100% O₂ during general anesthesia. The second group consisted of five persons as same criteria as above. The patient was anesthesized as same as above methed and the patient was hypoventilated for sometimes during anesthesia and arterial blood gas checked before and after hypoventilation. The results were as follows: 1) Under general anesthesia with 100% O₂ and halothane, MEP decreased. The range of the minimal MEP was from -40 to -255 daPa (1 daPa=1.02 mm H₂O). 2) Before the hypoventilation during anesthesia, the mean value of MEP was -83 daPa, and PCO₂ was 28.0 mmHg. 3) After the hypoventilation during anesthesia, the mean value of MEP was -49 daPa, and PCO₂ was 39.7 mmHg. Based on the results, lowering PCO₂ by general anesthesia decreased the MEP, and raising PCO₂ by hypoventilation caused elevation of the MEP. So the MEP was affected by PCO₂ changes of the blood.