Abstract: Method and apparatus for grinding pulp stock in the grinding space of a defibrating apparatus, which space is defined between grinding surfaces carried by a pair of grinding discs, both of which rotate relative to one another within a closed housing, to thereby control the rate of flow of grist through the grinding space. The grinding space includes an inner grinding zone and two outer grinding zones which diverge at an angle to the radial plane of the inner zone. The inner grinding zone is defined between the two facially-opposed rotating discs in a plane substantially perpendicular to the axis of rotation, and the outer grinding zones are defined between portions of the two rotating discs which extend at an angle to the radial plane of the inner zone and correspondingly inclined facing surfaces of a stationary element such as a stator ring encasing the two rotating discs.