An on-line technique is described for measuring aortic compliance using a two-probe Doppler ultrasound method. It is shown that automatic processing of blood velocity signals obtained simultaneously from the left subclavian artery and abdominal aorta enables real-time computation of the time delay observed between the systolic upstroke (or foot) of sonograms from these arteries during the same heart beat. An average foot-to-foot transit time (obtained from a number of cardiac cycles) and the physical separation between the two ultrasonic transducers is used to calculate aortic compliance. Using the above method, the reproducibility of aortic compliance measurements was studied in 12 healthy volunteers over different time intervals. Four of these subjects were measured 7 times each at 10 min intervals whilst the remaining eight subjects were measured 9 times each at hourly intervals. This reproducibility study indicated a significant variation of aortic compliance (P less than 0.001) in the same person regardless of time period. The coefficient of variation ranged approximately between 5% and 18%.