Electric self-potentials (SP) and strains were measured on the surface of a pillar of the abandoned underground Meriel limestone quarry (France). The time variations of strains and SP are interpreted in terms of crack instability. The dynamic mapping technique shows that the most intensive SP variations, corresponding to maximum stress variations estimated from strain data, occur in the vicinity of active crack tips or crack intersections. The location of these maxima changes with the stress distribution variations. The synchronous stacking technique allows to detect significant pressure variations through electrodes installed in the fractured zone as well as through outside electrodes close to the medium discontinuities (petrological boundaries and pillar edges). ¿ 1999 American Geophysical Union