1. A method of measuring intravascular temperatures in anesthetized dogs has been described.

2. The temperature in the abdominal aorta is uniform throughout, and varies only with the systemic temperature.

3. The temperature in the inferior vena cava rises as the thermo-couple approaches the heart, reaching its maximum at about the level of the hepatic veins. Between the hepatic veins and the right chambers of the heart there is no further elevation in venous temperature.

4. The temperature of the right heart blood normally exceeds that of the left heart blood by 0.05–0.2°C.

5. During the application of high frequency currents to the thorax, this relationship is reversed.

6. This indicates that the lungs are being heated but that the blood passing through the pulmonary vessels is removing the heat at approximately the rate of production.

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