A ring binder mechanism for retaining loose-leaf pages comprises a housing and first and second hinge plates supported by the housing for pivoting movement. Ring members are mounted on the hinge plates for movement with the hinge plates between closed and open positions. Hook-shaped formations on fee ends of the ring members lock the ring members together when closed. The first hinge plate is slidable relative to the second hinge plate in a direction along a longitudinal axis of the housing for disengaging the hook-shaped formations and allowing the ring members to open. An actuator may be mounted on the housing for moving the first hinge plate to disengage the hook-shaped formations. In addition, friction buffers may be positioned between the hinge plates for aiding the sliding movement of the first hinge plate relative to the second hinge plate.