Nitrous oxide is the most commonly used inhaled anesthetic in the part of anesthesia and used up to 75% of concentrations. Diffusion hypoxia among the disadvantages or harmful damages due to nitrous oxide exposure must be prevented by moderate flow (4-6 liters/minute) of oxygen for a few minutes. This study was investigated the effect of the amount of oxygen flow on the speed of removal of exposed nitrous oxide followed by oxygen flow rate of 2, 4, and 6 liters/minute when halted the administration of nitrous oxide. These variables were taken in 57 patients of 16 to 60 years old, who were performed the elective surgery. All patients were anesthetized with the 0.5-1.5 MAC of enflurane or isoflurane combined with nitrous oxide(2 liters/minute) and oxygen(2 liters/minute), and paralyzed with IV route pancuronium 0.07-0.08 mg/kg. Ventilation was controlled with Ohmeda 7000 ventilator (BOC Health Care Inc, Madison, USA), using a constant tidal volume of 10 ml/Kg of ideal body weight. Ventilatory rate was adjusted 12 times/minute to maintain the end-tidal CO2 of 20-35 mmHg. After 60 to 90 minutes of anesthesia, the nitrous oxide/oxygen mixture was changed to 100% oxygen, but ventilation being held constant. The results were as follows; 1) After the first 30 seconds, the end-tidal nitrous oxide concentration was 39.6±+3.7% in 2 liters/minute of oxygen flow, 28.2±5% in 4 liters/minute and 23.4±6.3% in 6 liters/minute. 2) After the 2 minutes, the end-tidal nitrous oxide concentration was 29.1±3.6% in 2 liters/minute of oxygen flow, 14.4±3.2% in 4 liters/minute and 10.13±2% in 6 liters/minute. 3) After the 5 minutes and 30 seconds, the end-tidal nitrous oxide concentration was 16.4±3.3% in 2 liters/minute of oxygen flow, 5.5±1.9% in 4 liters/minute and 4.0±1.7% in 6 liters/minute. 4) After 15 minutes, the end tidal nitrous oxide was 7.5±2.1% in 2 liters/minute of oxygen flow, 2.3±0.7% in 4 liters/minute and 2.0±0.8% in 6 liters/minute. In conclusion, the larger size of oxygen flow, the more rapid elimination of nitrous oxide. The removal rate of nitrous oxide was greatest at first 30 seconds after halting the nitrous oxide administration in all cases.