Changes in permeability have been studied during the flow of water through granite for periods of time up to 17 days at temperatures of 100¿, 200¿, 300¿, and 400¿C with a constant confining pressure of 500 bars, differential stresses of 0--3500 bars, inlet pore pressure of 275 bars, and outlet pressure of 1 bar. In all cases the initial permeability at elevated temperatures was found to be higher by 1--2 orders of magnitude than the permeability at room temperature, perhaps because of thermal stress cracking. The high initial permeability did not persist with time and in nearly all cases decreased significantly during the first 1/2 day of water flow. Dissolution of plagioclase and quartz was concentrated near the inlet, where the pore pressure was highest. Precipitation occurred throughout the samples because of oversaturation as the pore pressure dropped, causing significant reduction in permeability. The final permeability after 10 days was less at 300¿C than at lower temperatures, and measurable flow stopped in most of the 400¿C samples.