Complete consciousness may return Tery slowly following general anesthesia. Although most patiients appear to approach their preanesthetic state within minutes to tours, but there appears to be some changes in cognitive functions for the next 2 to 4 days after halothane anesthesia. For the evaluation of recovery time from halothane anesthesia, we studied 10 pediatric patients who received N2O-O2-halothane anesthesia, During the recovery after cessation of halothane inhalation, we administered N2O (2 l/min) and O2 (1 l/min) with Dameca anesth-esia machine. All the patients ware ventilated with Dameca ventilator. The minute ventil-ation was 145 ml/kg/min. Throughout the experiments, we measured inspired( l) and end-tidal (ET) O2, CO2, N2O, N2, and halothane concentrations with mass spectrometry. Control data were obtained just after cessation of halothane inhalation, and all the data after then were compared with control data and expressed as% response of control. The concentrations of O2, CO2, N2, and N2O were net changed significantly during the experiment. The inspired halothane concentration (C1 HAL) in control was 1.14±0.08 vol% and it sho- wed significant progreasive decrease by each minute, The most abrupt decrease in C1 HAL was noticed in 3 min and 6 min: 68.8% and 73.0% in 3 min and 6 min respectively. The endtidal halothane concentrations (CET HAL) were 1.06 vol%, 0.52 vo1%, 0.39 vol% and 0.32 vol% in control, 3 min, 6min and 9 min respectively. The end-tidal halothane concentrations were also significantly decreased progressively. CET HAL/C1 HAL ratio was 0.91±0.02 in centrol, but it wag alwayg more than 1.0 after cessation of halothane, which indicates continuous elimination of halothane from the body, The complete disappearance times of inspired and end-tidal halothane calculated from the linear regreasion equations were 60.26 min and 63.10 min, respectively.