In nine anesthetized dogs, recordings of the first derivative of the transthoracic impedance cardiogram (ICG) were made during varying grades of acute aortic regurgitation. Acute aortic regurgitation was induced using a specially designed umbrella catheter, passed retrograde across the aortic valve into the left ventricle. The RFA (representing the fraction of the aortic reverse flow to the aortic forward flow) was computed using an electromagnetic flow probe implanted around the ascending aorta. Both the peak of the scalar ICG, dz/dtmax, which occurs at peak systolic ejection, and the nadir of the scalar ICG, X, which marks the closing of the aortic valve, increased with aortic regurgitation. The planimetered areas of the ICG during systole (S), and in early-diastole (X) increased during aortic regurgitation. These areas, S and X, correlated with the electromagnetic normalized aortic stroke volume (r = 0.90) and the regurgitant volume (r = 0.78), respectively. The ICG ratio X/S was correlated directly with the electromagnetic aortic regurgitant fraction (r = 0.86). This study demonstrates that the ICG waveform is consistently modified by experimental aortic regurgitation. Furthermore, these changes can be quantitatively related to the degree of aortic regurgitation.