The effects of steel and tungsten carbide burs have been studied under the scanning electron microscope on human enamel and dentin during preparation of mesio-occluso-distal cavities in vitro with cutting speeds varying from 5 000 to 120 000 r.p.m. All the cavities presented clean and regular walls. However, the final quality of the cavity borders, and in particular the cavo-surface angles of the box walls, was directly related to the speed and rotation direction of the bur. The cavo-surface angles at the gingival planes showed in all cases fractures of the superficial prisms. The cutting character displayed in dentine surfaces were mainly dependent on the orientation of the tubules in relation to the surface of the cavity preparation and the degree of sclerosed dentine. The cavities prepared in the absence of water and air showed the presence of tissue debris nd cracks; their number increased with increasing rotational speed.