Anesthesia was induced with intravenous thiopental, fentanyl, and succinylcholine, and maintained with isoflurane, N2O-O2 and atracurium after denitrogenation in 20 adult patients. Inspired and end-tidal concentrations of O2, N2O, CO2, isoflurane and oxygen saturation of pulse oximeter (SpO2) were monitored. After intubation, N2O (750 ml/min) and O2 (250 ml/min) were administrated. When inspired N2O concentration (FiN2O) reached 60% the closed anesthesia was initiated by adjusting fresh gas flow rate in order to meet the oxygen demand of the patients. The N2O/O2 ratio was constantly kept at 0.7. The results revealed that during wash-in period, FiN2O increased, on average, to 30% at 3.83 min, 40% at 6.22 min, 50% at 14.13 min respectively. After 30 min of closed anesthesia, FiN2O decreased to 49.4%. It was 46.8% at 60 min. Thereafter it increased smoothly to 51.9% at 180 min. No hypoxia and hypercapnia occurred. Compared with commonly used high flow N2O-N2 inhalation, the technique of low flow wash-in and closed circuit maintenance with N2O-O2 may be less expensive and feasible and causes less pollution.