Co-seismic signals of geoelectric potential have been observed for several earthquakes (EQs) in Japan. It has been argued that if there are precursory changes, stronger signals should be at the time of EQ. Our results prove that co-seismic signals do exist, but they start with the arrival of seismic waves and not at the origin time of EQs. Their amplitude does not scale with the dipole length. Both facts indicate that the changes are local effects of passing seismic waves and not due to electric signals emitted from the seismic source. Observed changes contain two components; oscillatory and offset/decay ones. The former is synchronized with seismic waves, whereas the latter, with a rise time of a few seconds and a decay time of 0.1--1 hour, is suspected to be related to EQ induced subsurface hydrological change. In both cases, the origin of electric potential variations may be electrokinetic. ¿ 2000 American Geophysical Union