The outer diameter of the vessel and the blood velocity are measured simultaneously by means of the ultrasonic transit-time technique and the ultrasonic Doppler method, respectively. Instantaneous blood flow is calculated by multiplying the flow velocity by the cross-sectional area calculated from the diameter. Stroke volume and cardiac output can also be obtained when the system is applied to the ascending aorta. After preliminary benchtop tests, acute in vivo studies were carried out to evaluate the accuracy and performance of the system. The blood flow in the descending aortas of dogs and a goat were measured simultaneously by the ultrasonic system and an electromagnetic flowmeter. The correlation coefficient between the data obtained by the two methods was 0.968.