A blood pressure can be obtained by supplying an external pressure to a portion of an artery. The external pressure is preferably between the systolic and diastolic pressure. An event which occurs at least once a cycle can then be identified. This event can be, for example, a peak in the arterial compliance that occurs at a transmural pressure approximately equal to zero. A pair of signals, one that is an arterial volume-indicating signal, and one that is an arterial pressure-indicating signal can be used to identify this event. Alternately, a small signal high-frequency exciter component can be placed upon the pressure or volume of the artery and detected to determine the time that the transmural pressure is equal to zero.