Utilizing the cardiac catheterization technique, investigated the cardiopulmonary functions in 15 cases out of the 17 patients underwent surgery for patent ductus arteriosus and obtained the following results. Cardiac Functions 1. Both the pulmonary arterial pressure and the total pulmonary vascular resistance were either normal or slightly increased with a few showing a high degree of increase in them, but there was a correlation between the two; and also the crosss-sectional area of the ductus arteriosus was correlated to both the pulmonary blood-flow and the mean pulmonary arterial pressure, with exception of cases where total pulmonary vascular resistance was markedly elevated. 2. A correlation could be recognized between the pulmonary blood-flow ratio and the shunt-flow ratio, and the shunt-flow was mostly directed from left to right. 3. In cases where the cross-sectional area of the ductus was over 0.5cm2, many of them were clinically in advanced stage, and this area of the cross-section together with the total pulmonary vascular resistance were the important factors controlling the cardiac functions of this disease. 4. The ventricular works tended to increase in both the right and left, and the hypertrophy of the both ventricles observable in electrocardiogram is indicative of the condition gradually leading to cardiac failure. Pulmonary Functions Observations were carried mainly on pulmonary circulation and ventilation. 1. Minnte ventilation volume was either normal or slightly inccreased. In cases, with pulmonary hypertension, pulmonary blood-flow of over 10l/min., shunt-flow of over 5l/min., or with cross-sectional area of the ductus of over 0.5 c㎡, the minute ventilation volume was incresed. 2. Maximal breathing capacity was either normal or mildly decreased. There was an inverse correlation between the maximal breathing capacity ratio and pulmonary arterial pressure in the case either with pulmonary hypertension or with an increased area of cross-section of the ductus. 3. No decrease could be observed in vital capacity ratio. There were inverse correlations between vital capacity and various factors of circulatory functions. On the other hand, there was a positive correlation between vital capacity and maximal breathing capacity ratio. 4. Oxygen equivalent showed a slight increase, and those cases showing the oxygen equivalent of over 4.0 were all severe. 5. Ventilatory reserve and ventilatorg reserve ratio were normal, and those showing ventilatory reserve ratio under 70 per cent were all inoperable cases. 6. The oxygen equivalent upon exsercise did not show any diffeence with compared to that at rest, showing findings different from those of cyanotic congenital heart diseases.