Background : N2O has a unique analgesic effect and reduces the amount of hypotics for preventing surgical stimuli during maintenance of anesthesia. Also, it was reported that high concentrations of N2O affect level of consciousness. The aim of this study was to evaluate the effect of inhaled concentrations of N2O during emergence on awakening time after propofol-N2O-O2 anesthesia. Methods : Sixty ASA class I or Ⅱ patients scheduled for lower extremity surgery were randomly allocated to one of three groups according to inhaled concentration of N2O during emergence. Group 1 : 0% N2O (n = 20), group 2: 33% N7O (n=20) and group 3: 50% N2O (n=20). Anesthesia was induced and maintained with propofol (Ct : 3.5-6 ug/ml)-67% NBO-33% O2 and the target concentratirln of propofol was kept at 4 ug/ml at least 30 min before the end of infusion of propofol using TCI. At the time of skin closure, we discontinued the propofol, mairltained the allocated concentration of N2O and continuously checked vital signs,current/effect concentration of prorofol, bispectral index (BIS), and elilpsed time until eye opening to verbal contact (awakening time). Results : Awakening time and bispectral index significantly increased as the inhaled concentration of N2O was higher. At awakening time, The predicted current/effect site concentrations of propofol sign-nlificantly clecreased as the inhaled concentrations of N2O were increased. Conclusions : Continuous inhalation of N2O after disoontinuation of propofol infusion significantly delayed the awakening time after propofol-N2O-O2 anesthesia using TCI. (Korean J Anesthesiol 2000 : 35: 205∼211)