Pulmonary function changes during and after cardiopulmonary bypass were studied using single breath test for CO_2 (SBT-CO_2) in 16 adult patients undergoing cardiac surgery. The results showed that P_(A-a)CO_2 and P_(A-a)O_2 increased significantly after bypass though PaCO_2 kept in normal range by adjusting ventilation volume. The CO_2 production increased as time passed after bypass, resulting in the increase of required minute volume and the rise of airway pressure. Compliance showed a tendency to decrease, while physologic dead space and alveolar dead space increased significantly. It is concluded that the causes of pulmonary dysfunction occurring in early stage of post-bypass are mainly due to the V/Q mismatch induced by low perfusion of the lung. To improve the pulmonary function at the early stage of post-bypass, the circulatory function should be improved accordingly.