In the 1st two years (From April 1975 to March 1977), we have had experience with 15 surgical infant patients undergoing general inhalation anesthesia with the "Loasco" Infant Ventilator. The "Locsco" Infant Ventilator is designed for the ventilation of neonates and infants on the lines of an open system with a gas mixture of air-oxygen or oxygen-anesthetics. However we used it with a gas mixture of halothane-N₂O-oxygen, This study was undertaken to evaluate the value of ventilatory support with the "Loosco" infant Ventilator during general inha!ation anesthesia for 15 surgical patients whose age was less than 2 years of age. The blood gas analysis showed slight under ventilation combined with metabolic acidosis during the first 15 minutes, in which respiratory frequency was set at 30 times per minute, but the ventilation was gradually improved by increase in respiratory frequency to 35 times per minute during the lost 20 minute. In this study, we considered. the important factors which influenced the value of the blood gas analysis. Not only careful anesthetic management to keep the airway and to convect cardiovascular derangement, but also frequent and repeated blood gas analysis and its accurate interpretation were stressed for using the "Loosco" Infant Ventilator during pediatric general anesthesia.