After the establis banent for the necessity of denitrogenation in rebreathing anesthesia circuits some decades ago, many techniques were recommended for venilation with oxygen before induction of anestesia for the purpose of maintaining arterial oxygenation during laryngoscopy and tracheal intubation. For this point of view, we compared PaO2, MAP, PaCO2, pH of non-preoxygenation group (oxygen was given after succinylcholine was injected) with those of preoxygenation group (oxygen was given after thiopental sodium was injected). The cases were selected randomly ASA class I or II (without cardiopulmonary abnormalities) and ages between 27 years and 66 years old. Before induction of general anesthesia, we cannulated the radial artery after Allen's test and sampled arterial blood before anesthesia, after thiopenthal and succinylcholine were injected respectively, and when tracheal intubation was done in the two groups and measured PaO2, PaCO2, MAP, pH respectively. PaCO2, MAP, pH changes in the two groups were not significant and of no clinical significance. PaO2 after tracheal intubation showed a marked increase in the preoxygenation group compared to the non-preoxygenation group but the mean PaO2, remained within normal range in the non-preoxygenation group with the apneic period of taecheal intubation. We found that non-preoxygenated patients also had a normal range of PaO2 during the whole process of induction of anesthesia. But we think preoxygenation technique of any method can reserve more time and will do more effeclively, especially when time consuming events of difficult intubation of any reason occur.