Thirty (19-58yr) patients received halothane, enflurane and isoflurane with Ohmeda, Cyprane and Narcomed vaporizer for maintenance of anesthesia during controlled ventilation. End-tidal fractional concentrations(FE) of the volatile anesthetic agents were measured for 40 minutes after induction. Inspired oxygen fractional concentrations during anesthesia in the three groups were slightly decreased from minimal 33.1±2.3% to maximal 49.3±1.7%. End-tidal carbon dioxide concentrations during anesthesia in the three groups were within normal lirnit ranged from minimal 24.5±3.2 mmHg to maximal 35.2±3.4 mrnHg. When vaporizer's dial was fixed at 1% halothane, average measured FE/FI ratio were gradually increased from 5 minutes to 40 minute in the three groups. But FE/FI ratios were lower than dial setting, except after 40 minutes in the Narcomed vaporizer. When vaporizer's dial were fixed at 1% isoflurane and enflurane, average measured FE/PI ratios in the three groups were nearly equal but lower than dial setting. These results suggest that the actual gas concentrations of used(2-3yr) vaporizers were measured slightly lower than dial setting and we need to continuously monitoring of respiratory gas and end-tidal anesthetic gases.