The development of endotracheal tubes with high-volume and low-pressure cuffs has decreas ed the incidence of associated tracheal injury. If room air used for inflation of the cuff during general anesthesia using N2O-O2, mixtures, however, endotracheal tube cuff result in distention and potential pressure changes. Therefore, tracheal pressure injury may occur during long general anesthesia(especially, head and neck surgery). For the prevention of this problems and the determination of proper methods, we performed this study relating to nitrous oxide diffusion into a gas filled endotracheal tube cuff.