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In order for the conductance catheter system to accurately measure instantaneous cardiac blood volume, it is necessary to determine and remove the contribution from parallel myocardial tissue. In previous studies, the myocardium has been treated as either purely resistive or purely capacitive when developing methods to estimate the myocardial contribution. We propose that both the capacitive and the resistive properties of the myocardium are substantial, and neither should be ignored. Hence, the measured result should be labeled admittance rather than conductance. We have measured the admittance (magnitude and phase angle) of the left ventricle in the mouse, and have shown that it is measurable and increases with frequency. Further, this more accurate technique suggests that the myocardial contribution to measured admittance varies between end-systole and end-diastole, contrary to previous literature. We have tested these hypotheses both with numerical finite-element models for a mouse left ventricle constructed from magnetic resonance imaging images, and with in vivo admittance measurements in the murine left ventricle. Finally, we propose a new method to determine the instantaneous myocardial contribution to the measured left ventricular admittance that does not require saline injection or other intervention to calibrate.