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The cardiac output flow pulse (less coronary flow) can be recorded with a magnetic flowmeter applied to the unopened ascending aorta, provided: 1) the extremely large EKG potentials in this region are rejected, 2) the flowmeter is phase-sensitive, and 3) the over-all instrument response is uniform from zero to 100 cps. These requirements are fulfilled by the instrument described through the use of the square-wave method recently introduced by A. B. Denison, and by using a high switching frequency (480 cps), an input high-pass filter, and a double-balanced demodulator. Two output channels of suitable response provide for simultaneous recording of instantaneous and mean flow. Each pickup sleeve is calibrated in vitro in terms of microvolts per flow rate. An electrical series calibrator provides the operational calibration. Past methods are reviewed and circuit details and design considerations are discussed.