The nitrous oxide anesthesia is known to induce middle ear pressure variations. The middle ear pressure begin to rise immediately after the inhalation of nitrous oxide. The mechanism of this increase is known to be due to the difference in blood solubility between nitrous oxide and nitrogen. Repeated cycle of gradual rise and sudden fall of the pressure occur during anesthesia due to the diffusion of nitrous oxide and passive venting of the eustachian tube. After cessation of nitrous oxide administration, the gas diffuses out of middle ear faster than nitrogen diffuses in. Consequently, the total pressure falls producing the negative pressure within the middle ear. The present study was undertaken to investigate if such changes in middle ear pressure also occur during anesthesia with low concentration of nitrous oxide. In 30 patients without ear diseases, the middle ear pressure was measured with an impedance audiometer(Danplex C 186, Danamedica, Copenhagen, Denmark) during anesthesia with 50% nitrous oxide. We observed a significant increase in the middle ear pressure during the administration of nitrous oxide and a significant decrease after nitrous oxide was discontinued. These results indicate that nitrous oxide inhalation induced a significant change in the middle ear pressure even in a low concentration, thus we advice that the administration of nitrous oxide be ceased at least fifteen minutes before the middle ear cleft is closed by grafting a tympanic membrane defect or replacing a tympanomeatal flap.