A ring binder mechanism that retains loose-leaf pages and has ring members that are easy to open and close and that readily lock together when closed. The mechanism comprises a housing that supports two hinge plates for loose pivoting motion. The hinge plate motion moves the ring members between an open position for loading pages and a closed position for retaining pages. A control structure is movable by a lever, which is pivotally mounted on the housing, between a first and second position. In the first position, pivoting motion of the hinge plates is blocked, and in the second position, the hinge plates can freely pivot. In one aspect of the invention, a spring is engageable with the lever for urging the lever to move the control structure toward the first position.