Permeability is the critical factor governing fluid flow in accretionary prisms. Accretionary wedges are highly deformed, so permeability changes in an indurated claystone sample from the Nankai accretionary prism were measured during a triaxial stress experiment by the pulse decay method. Three zones were identified from the loading test. In zone I, the sample has deformed less than 1.3% and its permeability was 5.10-20 m2. In Zone II, the deformation reached 1.5% and the permeability was unstable with time. In zone III, the sample deformation reaches 2% and its permeability reached 3.8⋅10-18 m2. The permeability strongly decreased (from 3.2⋅10-18 m2 to 1.7⋅10-19 m2) with increasing effective pressure (from 2.3 to 10.8 MPa) after the sample was well deformed (zone III). The streaming potential was not measurable when the sample had a low permeability (zone I), but clearly occurred when the permeability increased (zone III): the streaming potential measured was 6 mV when the pore pressure pulse applied was about 1.17 MPa and the permeability 3.8⋅10-18 m2. Our experiments suggest that shear deformation under low effective pressure increased the vertical permeability of sediments above the d¿collement. This increase in permeability may be detected by measuring the streaming potential. Variations of flow rate of expelled fluid in accretionary wedges may be detected by monitoring changes of electrokinetic potential, giving new insights on the state of stress related to the seismic cycles. ¿ American Geophysical Union 1994