Preoxygenation is routine preior to induction of general anesthesia for the purpose of maintaining oxygenation during laryngoscopy and tracheal intubation. A common method of preoxygenation is 3~5 minutes of 100% O2 breathing. In some emergency states, ther may not be adequate time for a full 3~5 min of preoxygenation. REcently, Gold and some others showed that four maximally deep inspirations of 100% O2 within 30 seconds are as effective as 5 minutes of inhaltion of 100% O2 for increasing the PaO2. We compared the changes of arterial blood gas between a group which took deep breaths for 1 minute with 100% O2 and a group breathing spontaneously for 5 minutes with 100% O2.