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The amplitude and phase changes of ultrasound echoes from biological tissues are investigated when a physiological-level (< 2.0 V/cm) DC or extremely low frequency AC electric field is applied to the tissues. Ultrasound echo signals were acquired continuously while the transducer was fixed at one position during the application of the electric field. Some small windows are chosen in the ultrasound signals to analyze the amplitude and phase changes. The peak-to-peak values of the signals are used to represent the amplitudes, and the flight times of the peaks are used to represent the phases in each individual window. Experimental results show that the amplitude changes and the phase shifts of the echo signals are correlated with the applied electric current. The direction of the amplitude changes but not the phase changes is reversed when the direction of the electric field is reversed. More studies are necessary to understand the underlying mechanisms of the observed changes. The reported results suggest a new method to monitor the changes in biological tissues due to the electric field and a new contrast mechanism for ultrasound imaging.