Intracardiac and intravascular blood temperatures were measured in a group of 17 afebrile patients undergoing cardiac catheterization. Using a cardiac catheter with a thermistor mounted at the tip, measurements were made in the following locations: different levels of the inferior vena cava, the superior vena cava, the renal, hepatic, and internal jugular veins, the right atrium, pulmonary artery and pulmonary artery wedge position, coronary sinus and right ventricle, and the left atrium and pulmonary veins (in subjects with atrial septal defects or patent foramen ovale). Data obtained confirm and extend observations made by other investigators. The blood temperature in the pulmonary artery, pulmonary artery wedge, left atrium, and pulmonary vein were found to be very nearly the same. Furthermore, temperature recordings made in different sites of the inferior vena cava, superior vena cava, right atrium, and pulmonary artery show variations phasic with respiration. The mechanism of these thermal variations is discussed. Submitted on February 5, 1962