Abstract: A vehicle roof mounted ventilator having a support, a cover and an operating mechanism for selectively moving either the front or rear of the cover between open and closed positions. The operating mechanism includes a pair of links pivotally connected to the cover at one end and to a cam follower at its other end, a horseshoe shaped cam for slidably receiving the cam followers and a rotatable knob for rotating the cam in opposite directions to selectively effect movement of one of the links while the other remains stationary due to its associated cam follower riding in the bight portion of the horseshoe shaped cam.