In cardiac surgeries, it is frequently necessary to make the external blood circulation and oxygenation artificially. This procedure is called extracorporeal circulation (EC) or heart-lung bypass. During EC, one of the most important parameters, which demands continuous monitoring, is the blood flow. In many cases, the blood flow is estimated by the pump velocity (in roller pumps) or measured with transducers based on electromagnetic methods (in centrifugal pumps). This article presents an ultrasonic Doppler blood flowmeter to be used in the arterial line of an extracorporeal circulation system. The ultrasonic probe is coupled to a half-inch tube connector of the extracorporeal arterial line, is not disposable, and does not need sterilization. The developed flowmeter revealed itself to be efficient and reliable and can be inserted in a closed-loop pump controller system or be used as stand-alone equipment.24198-201