Fluid flow at hydrostatic pressure (Ph) is relatively common through fractures in silicic and in mafic crystalline rocks where temperatures are less than about 350--370 ¿C. In contrast, pore-fluid pressure (Pf)>Ph has been encountered at the bottom of 3 geothermal exploration wells that attained temperatures >370¿C (at Larderello, Italy, at Nesjavellir, Iceland, and at The Geysers, California). Chemical sealing by deposition of minerals in veins appears to have allowed the development of the high Pf encountered in the above wells. The upper limit for the magnitude of Pf that can be attained is controllyed by either the onset of shear fracturing (where differential stress is relatively high) that reopens clogged veins, or the hydraulic opening of new or old fractures (at relatively low values of differential stress). The brittle-plastic transition for silicic rocks can occur at temperatures as high as 370--400 ¿C in tectonically active regions. In regions where high-temperature geothermal systems develop and persist, it appears that either strain rates commonly are in the range 10-12 to 10-13, or that silicic rocks in the shallow crust generally behave rheologically more like wet quartz diorite than wet Westerly granite.