The electrical resistivities of several dacitic tuffs, sandstone, andesite, granite, and crystalline limestone samples, saturated with a 0.001 M aqueous solution of KCl, were measured in the range from room temperature to 250 degrees C. The experiments were made using a cell technique with platinum electrodes. Of particular interest are the data collected at temperatures above 200 degrees C under high pressure, a region scarcely documented in the literature.Basically the samples used in the present study show a quasi-exponential decrease of resistivity with temperature up to 200 degrees C. The same temperature dependence is found for the resistivity of the saturating solution, thus confirming that conduction in water-saturated rocks is essentially electrolytic. At temperatures above 200 degrees C, some specimens of highly porous dacitic tuff still closely follow the saturating solution in the pattern of resistivity variation with temperature, exhibiting a minimum around 220 degrees C; however, the behavior of low-porosity crystalline rocks (notably granite) where a relatively abrupt decrease of resistivity is observed above 50 degrees C, departs from that of the saturating solution. Hysteresis phenomena are more or less observed in most rock samples; i.e., the resistivity after one complete thermal cycle is systematically lower than its initial value. This experimental evidence points out that mechanisms different from water characteristics, such as growth of microcracks or chemical reactions, contribute to electrical conduction at high temperature. We point out the electrical signature of the thermally induced growth of micro-cracks (thermal cracking) in welded tuff and granite and suggest the possibility of using electrical measurements to monitor an extension of reservoir fractures in hot dry rock.