A numerical technique is presented to investigate seismic velocity changes in brittle rock due to stress and damage. A rock sample is simulated by an assembly of bonded particles, which is then subjected to hydrostatic and deviatoric loading regimes. During hydrostatic loading, cracks close through formation of new contacts and velocities increase. During deviatoric loading, new cracks form by the breaking of bonds and velocities decrease in directions perpendicular to the opening cracks. The nature of the velocity changes mimics closely what is observed in the laboratory. The model offers the unique ability to examine directly the microprocesses leading to observed velocity changes.