The arterial carbon dioxide partial pressure (Pco2) & pH are basic indices of the adequacy of ventilation in anesthetized patients. Obstacles to the use of these indices intraoperatively are the inconvenience and risks associated with obtaining arterial blood, especially in the infant. Venous blood samples may be obtained conveniently, repeatedly and without additional hazard. If venous rather than arterial blood could be used for indirect estimation of pH and PaCO2, it would encourage the use of such measurements. Venous blood drawn from back of the hand or distal third of forearm is derived principally from cutaneous blood flow, which is usually increased by general anesthesia or local heating. This study compares the values for Pco2, pH and O2 saturation of blood samples drawn from a radial or femoral artery and a vein on the back of hand during general anesthesia, preanesthesia and postanesthesia in recovery room.