We describe the first mapping of biological current in a live heart using Ultrasound Current Source Density Imaging (UCSDI). Ablation procedures that treat severe heart arrhythmias require detailed maps of the cardiac activation wave. UCSDI can potentially improve on existing mapping procedures which are time-consuming and limited by its poor spatial resolution (5-10 mm). UCSDI is based on a pressure-induced change in resistivity known as the acousto-electric (AE) effect, which is spatially confined to the ultrasound focus. Data from two experiments are presented. A 540 kHz ultrasonic transducer (f/#=l, focal length=90 mm, pulse repetition frequency=1600Hz) was scanned over an isolated rabbit heart, perfused with excitation-contraction decoupler to significantly reduce motion while retaining electric function. Tungsten electrodes inserted in the left ventricles recorded simultaneously the AE signal and the low frequency electrocardiogram (ECG). UCSDI displayed spatial and temporal patterns consistent with the spreading activation wave. The propagation velocity estimated from UCSDI was 0.25±0.05 mm/ms, comparable to the values obtained with the ECG signals. The maximum AE signal-to-noise ratio after filtering was 18dB, with an equivalent detection threshold of 0.1 mA/cm 2. This study demonstrates that UCSDI is a potentially powerful technique for mapping current flow and biopotentials in the heart.