Many attempts have been made to device a simple, yet accurate, laboratory method to evaluate the ventilation and acid-base disorders in patients. This study compares the values for Pco₂, pH and Po₂ of blood samples drawn simultaneously from a radial artery and a vein on the back of the other hand during general anesthesia in 62 cases. The mean arterial and mean venous Pco₂ and pH differences were 0.924 torr and 0.00013. These differences were too small to be of clinical importance and also varies slightly dependent on the main anesthestics used. Therefore we feel that the use of peripheral venous blood for pH and Pco₂ determination during general anesthesia is a reliable indirect method of arterial Pco₂ and pH.