HE MOST meaningful way of determining the mechanisms operative in the moment-to-moment control of ventricular functions is to obtain an accurate estimate of the performance of the heart in intact, unanesthetized animals. For this purpose, we should monitor exactly both static and dynamic cardiac effort.1 The important factor to be followed, therefore, is that of total cardiac work, both in normal, intact animals and in animals under a variety of circumstances. An estimate of the total tension developed by the heart is required when trying to follow the work from moment to moment. As has been previously stated, "The total tension developed by the heart can be accurately calculated when one knows the systolic residual and the end-diastolic volumes, the end-systolic and end-diastolic intracavitary pressures, the rate of change of volume and pressure from moment to moment during the contraction of the heart, and the K value of Laplace's Equation." 2 Our own studies have evolved from an attempt to monitor the instantaneous changes in the external volume of the left ventricle in intact, unanesthetized dogs by a technique of electromagnetic plethysmography.3 This method permits the continuous recording of the summated external cross-sectional areas of the left ventricle. The external volume of a chamber, such as the left ventricle, is directly related to, and varies with, the mean cross-sectional area of the chamber times its length. AVhere the changes in length of this chamber are extremely small, the instantaneous volume will be directly related to the sum of a series of its simultaneously and instan-