Objective: Address the two major shortcomings of classical conductance technology to determine left ventricular (LV) volumes in rodents. The first shortcoming is the incorrect assumption of a constant value for parallel conductance. The second shortcoming is the use of Baan's conductance to volume conversion equation, which incorrectly assumes a linear relationship between conductance and volume in the ventricle. Methods: We have used electrical admittance technology to determine instantaneous blood conductance in the ventricle by the removal of parallel conductance on a beat-by-beat basis. This blood conductance is then used in conjunction with a non-linear conductance to volume conversion equation (Wei's equation) to determine instantaneous LV volumes. Results: Real-time pressure volume loops were obtained from eight mice using the new admittance technology and were compared with the loops obtained using conventional conductance technology. Simultaneous 2D ultrasound images of the heart were also taken and used as a volume reference. Conclusion: We have shown that more accurate LV volumes are obtained using admittance and Wei's equation than conventional conductance technology and Baan's equation. The parallel conductance is shown to vary between end-systole and end-diastole and this variation is even more significant during IVC occlusions. We have also demonstrated that the increased accuracy of admittance in determining LV volumes extends to mice with aortic banding.