In postanesthetic period, infants and children have a risk of hypoxemia due to decreased functional residual capacity(FRC) and increased alveolar-arterial O2 tension gradient(A-a DO2). We measured arterial oxygen saturation(SaO2) with a pulse oximeter in 60 ASA class 1 infants and children. Group 1 was breathing with supplemental oxygen(4L/min) by mask and Group 2 was breathing with room air in recovery room after general anesthesia. SaO2 was measured on arrival in the recovery room, 2,4,6, 8,10,15, and 20 minute after arrival The results were as follows: 1) Postanesthetic SaO2 measured on arrival in the recovery room had decreased significantly(p<0.05) to preanesthetic SaO2 2) SaO2 measured in room air had decreased significantly compared with SaO measured in supplemental oxygen. As a result of the study, it is thought to be safe that supplemental oxygen is administered to patient on transfer and in recovery room.