The contour of the flow pulse along the normal unopened aorta of human subjects and dogs was established by recording with the surgical probe type, square-wave magnetic flowmeter. The contour at a given vessel segment was found to be the result of the transforming action of the arterial system on the ventricular ejection pulse. The characteristics of the aorta and its branches were found to be analagous to a low-pass electric tuned circuit, which resonates at 3 to 6 c.p.s. The resonant frequency, apparent from midsystole throughout diastole, was independent of heart rate, damped to extinction during diastole and renewed with each systole. New terms for elastic hydraulics are proposed and defined.