To examine whether CPB influences pulmonary vascular sensitivity to CO2, we compared the effect of slight induced hypocarbia and hypercarbia on pulmonary circulation before and after CPB in ten mechanically ventilated patients undergoing CABG. Hypocarbia was produced by increasing tidal volume slightly and hypercarbia was then induced by adding CO2 to the inspired gas mixture. In another ten patients, hypercarbia was produced after CPB by decreasing ventilator rate and the cardiopulmonary responses to hypercarbia, produced by the two methods of CO2 elevation, were compared. Slight respiratory acidosis induced by CO2 inhalation did not change PVR before CPB but effected a 50 percent increase after CPB. Hypercarbia induced by alveolar hypoventilation after CPB increased PVR by 40 percent. During the increased CO2 production after hypothermic CPB, pulmonary vasoconstriction would be expected to occur and impair right ventricular performance. Therefore, tight control of PaCO2 with appropriate adjustment of ventilatory support is mandatory.