From laboratory experiments, we observed that unloading stress on a piece of granite induced a pulsed electric potential variation, which decays exponentially with a time constant ranging from 1.5 to 4.0 s. The electric potential variation depends on the magnitude and the sustaining time of applied stress, indicating that the variation is due to piezoelectricity and migration of bound charges that compensate the piezo-electric polarization. An apparent piezoelectric coefficient of granite was obtained to be about 1.4¿10-15 C/N, three orders of the magnitude smaller than that of a single crystal of quartz. ¿ 1998 American Geophysical Union