This investigation was undertaken to determine whether venous blood, sampled under carefully controlled conditions, was an acceptable alternative to arterial blood for the measurement of arterial blood gas analysis. The arterial values for Pco2, pH, base excess and oxygen saturation were compared with the values of blood samples drawn simultaneously from the cephalic, external jugular and internal jugular vein during inhalation general anesthesia with 50% oxygen concentration in 25 cases. The results were as follows: 1) The blood gas analysis values of cephalic venous blood were closely comparable to those of arterial blood. There was no significant difference between the Pci2, pH and base excess of cephalic venous and arterial blood. 2) Although the oxygen partial pressure in cephalic venous blood was significantly less than that in arterial blood, the difference in oxygen saturation was small. 3) The blood gas analysis values of external jugular venous blood were between the cephalic venous blood and the internal jugular venous blood values. Those show that venous blood was arterialized and in general anesthesia, it's Pco2, pH and oxygen saturation will be near endough to those of the arterial blood. Although the oxygen partial pressure in venous blood was significantly less than that in arterial blood, the difference in oxygen saturation was small. Therefore arterialized venous blood from the cephalic vein may provide a reasonable estimate of presence or absence of hypoxia. in this study, we feel that the use of cephalic venous blood for Pco2, pH and oxygen saturation determination during general anesthesia is a reliable indirect method of arterial blood sampling.