This paper describes the first direct measurement of a cardiac activation wave using ultrasound. Arrhythmia caused by abnormal propagation of the activation wave is sometimes corrected with ablation procedures. The standard techniques to map the wave are slow and have limited spatial resolution (5 mm). The technique described here is based on the acousto-electric (AE) effect. The AE effect is a modulation of electric resistivity by ultrasound. When an ultrasound beam intersects a current field, a high frequency voltage is produced. In this study an isolated rabbit heart in a Langendorff setup was used. An excitation-contraction decoupler was used to eliminate motion while retaining electric function. The heart was paced from the apex, while two tungsten electrodes inserted in the left ventricles recorded both the AE signal, as well as the standard low frequency electrocardiogram (ECG). A 540 kHz transducer with f/#=l and focal length of 90 mm was focused in the heart and pulsed at 1600 Hz. Pulse echo (PE) and AE signals were acquired simultaneously, sampled at 12.5 MHz and averaged over 128 cycles. The AE signal appeared at the same time as the ECG signal and was extinguished with the ultrasound blocked. Its phase shifted by 180° when the transducer was moved up by half of a wavelength. Scanning the transducer in a line parallel to the long axis of the heart produced a time shift in the AE signal of 0.12 mm/s, a value similar to what has been reported previously. This study suggests that ultrasound current source density imaging (UCSDI) of the heart can potentially be used to map cardiac activation waves.