Laboratory experiments were performed to study the generation mechanism of electric signals during deformation and rupture of rock, with the focus on the effects of pore water movement. We have developed a triaxial apparatus that is specially designed for this purpose, within the framework of the International Frontier Program on Earthquake Research of the Institute of Physical and Chemical Research (RIKEN). This apparatus can deform a rock specimen that is electrically isolated from the surroundings at a very slow strain rate. Using a cylindrical specimen of intact rock (Inada granite, Makabe granite, Muroto gabbro, and Fujieda sandstone) saturated with distilled water, we carried out triaxial deformation tests under a constant pore pressure at room temperature. During a deformation test we continuously measured the electric current generated in the specimen. In addition, local strains, pore pressure, and pore fluid movement from the intensifier were recorded at a sampling frequency up to 1 kHz. The volume change due to dilatancy was obtained from the average of the local strain measurements at four positions. We found that the convection current flowed before the main fracture, showing good correlation with the dilatancy rate and the water flow rate. The current density of the signal was about 1 mA/m2. This result demonstrated that the electric current is caused by an electrokinetic effect due to the water flow associated with accelerating evolution of dilatancy before the fracture. ¿ 2001 American Geophysical Union