A valve mechanism is disclosed for a cylinder of an internal combustion engine which comprises two gas exchange poppet valves mounted in a cylinder head, a bridge 122 acting on the two valves, a cam for actuating the two valves, a slider 124 movable by the cam and slidable relative to the bridge 122, and a hydraulically controlled locking system having a latched position in which the slider transmits the force of the cam to the bridge 122 to actuate the two valves and a released position in which the valves are deactivated and the slider slides freely relative to the bridge. In the invention, the slider 124 is urged towards the cam by a spring 134 acting between the slider 124 and the cylinder head.