This study was attempted to observe the possible effects of open chest and the decubitus position on blood gas values during controlled ventilation. We used a volume-type ventilator to maintain regular tidal volume throughout the operation. A total of 51 cases which underwent open chest surgery between 1979 and 1981 in Department of Anesthesiology in Kyungpook National University Hospital were selected. PaO2, PaCO2, pH and MAP were measured at ten minutes after induction (control group), decubitus position (ducubitus group) and chest was opened(open chest group). The PaCO2 value in open chest and decubitus group revealed a tendency of elevation compared with the control group. The PaCO2 value in open chest revealed no remarkable change compared with the control group. The pH in open chest and decubitus group showed a tendency to increase compared with the control group. The MAP showed a tendency to decrease in open chest & decubitus groups and decreased gradually. From the above results, MAP showed a gradual decrease but the increase of pH and PaO2 suggested the inclination of respiratory alkalosis due to hyperventilation during volume controlled ventilation anesthesia.