An experimental method designed to measure ultrasonic velocities in simulated fault gouge subjected to normal and direct shearing stress yields these results. For a well-compacted, unsorted, fine-grained, dry granite gouge under constant normal stress, reversible changes in Vp with shear stress are observed prior to stable sliding. Vp decreases by as much as 9% with increasing shear stress for the conditions of these experiments. For layers of intact granite slabs subjected to constant normal stress, and whose interfaces are relatively free of gouge material, Vp increases with increasing shear stress. The observed velocity changes in the granite gouge experiments are believed to be produced by reversible opening and closing of small cracks. For a gouge material consisting of moderately compacted, sorted, coarser-grained, dry sand, irreversible changes in Vp were observed. The predominant changes in Vp for the sand gouge are believed to result from grain fracturing and from readjustment of grains to a denser state of aggregation with changes in the general stress.