Studying the cardiac functions in 11 patients and the pulmonary functions in 10 patients, with tetralogy of Fallot, the author obtained the following results. Cardiac Functions 1, The rise in the right ventricular pressure was more marked than that in the pulmonary arterial pressure and the systolic pressure of right ventricle more or less coincided with the systemic pressure. 2. Arterial oxygen saturation dropped down to 85-56 per cent and cases showing saturation below 70 per cent were clinically severe. The effective pulmonary blood-flow was 2, 017 cc/min., and those showing under 2, 000 cc/min. were all severe cases. Arterial oxygen desaturation and decreased effective pulmonary blood-flow were likely to be the most important factors evaluating the cardiac functions in this disease, and there was a correlation between the two. 3. Atrial septal defects were encountered in 6 patients out of 11. 4. Although the value above the normal level was maintained in the systemic blood-flow, it had no correlation with the effective pulmonary blood-flow. 5. In electrocardiogram incomplete right bundle branch block was revealed in the majority of the patients, and the right axis deviation or right hypertrophy were observed in almost half of the patients. 5. The average of right ventricular work was 4.85 kg. m/min./m2, and that of the left ventricular work 2.33 kg. m./min./m2; and an increase in the right ventricular work and a decrease in the left ventricular work were observed; namely, the right ventricular work was twice as much as the left. Pulmonary Functions 1. A marked decrease was observed in the volume of carbon dioxide in expired air, and this was due to hyperventilation. There was a positive correlation between arterial oxygen saturation and effective pulmonary blood-flow. 2. The increase in minute ventilation volume was more marked than that in acyanotic diseases. 3. A decrease in maximal breathing capacity ratio was observed; and maximal breathing capacity ratio was correlated to both arterial oxygen saturation and effective pulmonary blood-flow. 4. Vital capacity ratio was either normal or slightly decreased; and vital capacity ratio was correlated to both arterial oxygen saturation and effective pulmonary blood-flow. 5. There was observed a decrease in ventilatory reserve ratio, a marked increase in oxygen equivalent with an average of 4.6, and a poor ventilation efficiency. 6. Upon exercise test, minute ventilatory volume was markedly increased, and oxygen equivalent was increased as compared with that at rest, showing behaviors different from those in acyanotic diseases. The rate of the oxygen uptake was decreased, indicating that the increase in the pulmonary blood-flow upon exercise was restricted.