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The measurement of bidirectional bloodflow on a chronic basis is of critical importance in research on cardiovascular and pulmonary diseases. An implantable bidirectional Doppler ultrasonic bloodflow system was designed to satisfy this need. The system utilized quadrature detection to provide the directional information and multiplexing to transmit the quadrature signals over one FM telemetry link. A new family of three 1 V micropower bipolar integrated circuits were designed to implement the RF amplifiers, square law detectors, ±π/4 phase shifters, audio amplifiers, a multiplexer, and the FM transmitter required for this system. An implanted system was used to measure pulsatile bidirectional bloodflow on a canine descending aorta. The results compared well with those in the literature and with a percutaneous EM flowmeter.