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Classical investigation into the function and control of the heart has been conducted on anesthetized open-chested dogs. Unfortunately, both the anesthetics and the exposure of the heart affect cardiac function. Hence, more realistic information would be obtained if the heart could be studied in intact conscious animals. A system has been developed to make possible continuous analysis of the action of the heart in the healthy unanesthetized dog during its spontaneous activities. This system involves the continuous measurement of the pressure within the chambers of the heart, the size of these chambers, and the flow of blood out of the heart. Heart rate, stroke volume, average blood flow, effective cardiac power and work, and other information are continuously derived from the directly-measured parameters by means of analog computers. Several new instruments were developed to solve the problems unique to measurement in an intact animal. The dimensions of the heart chamber are obtained by measuring the transit time of pulsed sound passing across the chamber. Blood flow is measured by comparing the upstream and downstream transit times of bursts of sound passing through the moving blood. An isothermal flow meter utilizing a tiny thermistor on the tip of a catheter provides an alternate measure of flow. A miniature, differential transformer type of pressure transducer was developed for measuring pressure within a heart chamber. The system provides a means by which hypotheses regarding cardiovascular function and control may be rapidly and accurately evaluated.