This study was carried out to observe and evaluate the middle ear pressure(MEP) change by nitrous oxide(N2O) and end tidal PCO2 during general anesthesia with halothane or enflurane. MEP was measured during general anesthesia by impedance audiometer(GSI 28 Auto tymp model) in 50 patients who were relatively healthy without upper respiratory tract infection and otologic problems from August 1987 at Chonbuk National University Hospital. The results were as follows: 1) During general anesthesia with 100% O2 and halothane or enflurane, MEP was decreased progressively under the preinduction level with time. 2) The decrease in MEP during general anesthesia with 100% O2 and halothane or enflurane was reversed by relative hypoventilation, but MEP showed negative values in all patients. 3) End tidal PCO2 was increased progressively by relative hypoventilation and returned gradually to pre-hypoventilation level by normoventilation. 4) MEP was increased during general anesthesia using N2O, which is propotional to the concentration of N2O by 20 minutes. Thereafter, MEP remained increased until termination of N2O administration. From the above results, it is concluded that MEP is increased during anesthesia using N2O. MEP can also be affected by end tidal PCO2 even though physiologic range. Therefore, it is prudent to avoid N2O especially when hyperventilation is not adequately permitted during general anesthesia for middle ear surgery and patient with middle ear disease.