This study was attempted to observe the rate of fall of arterial oxygen tension and the rate of rise of artetrial carbon dioxide tension after denitrogenation with 100%(Group I, n=10) or 50% oxygen(Group II, n=15) in 25 healthy ASA class I patients scheduled for ear, oromaxillary, head and orthopedic surgery that do not affect respiration or pulmonary function. After 30 minutes of denitrogenation under supine position, apnea was carried out by dis- connecting the endotracheal tube and rebreathing circuit until arterial oxygen saturation decreased to 90-95% by pulse oximetry which was placed at index finger. We calculated the mean rate of decrease of arterial oxygen tension (PaO2(tn)-PaO2(tn+1)) and the mean rate of increase of arterial carbon dioxide tension (PaCO2(tn+1)-PaCO2(tn)) minute by minute by arterial blood gas analysis. The results are as follows. 1) The mean rate of decrease of arterial oxygen tension after apnea was 40.96+/-11.02 in Group I and 43.22+/-5.49 mmHg/min. in Group II 2) The rate of increase of arterial carbon dioxide tension during the first one minute of apnea was 5.94+/-0.85 in Group I and 5.56+0.64 mmHg in Group II 3) The mean rate of increase of arterial carbon dioxide tension after first one minute was 2.46+/-0.78 in Group I and 2.47+/-0.71 mmHg/min. in Group II With the above results, we concluded that healthy human subject who was denitrogenated with 100% oxygen about 30 minutes can withstand apnea as far as 7 minutes, and those who denitrogenated with 50% oxygen can withstand apnea as far as 3 minutes without hypoxic insult.