To study how the velocity dependence of steady state friction of granite varies as a function of sliding rate, we slid initially bare surfaces of Westerly granite at rates of from 0.001 &mgr;m/s (32 mm/yr) to over 3 mm/s, accumulating 376 mm of total displacement; the normal stress was 50 MPa. We find that the velocity dependence, measured by a-b, changes from negative (velocity weakening) at 32 &mgr;m/s. Modeling of experimental data using a simple constitutive law shows that the magnitude of a, the direct dependence of friction on velocity, remains nearly constant at all rates up to at least 100 &mgr;m/s. The increase in a-b with velocity is due to a reduction in the magnitude of b. Our results are consistent with the view that a measures the deformation rate sensitivity of the fault zone material and that b measures the amount of change of some fault zone property. ¿American Geophysical Union 1987