Blood pressure and flow were measured in the root of the aorta in conscious dogs. Aortic flow was varied by pacing the heart and by injecting myocardial stimulants into a coronary artery. The calculated aortic input impedance was capacitative in the lower frequency range and inductive above 3 to 6 cycles/sec. Graphical analysis also showed that aortic pressure rose by 50% of the pulse pressure when less than 1 ml. of the stroke volume had been ejected from the heart.