Abstract: A ventilation mechanism to be mounted in a window opening on a front lateral side of a motor vehicle, including a bracket adapted to be fixed in the window opening, a valve member supported on the bracket and movable between two positions for opening and closing ventilation aperture or apertures in the bracket, a rotor rotable through a predetermined angle in relation with the movement of the valve member, and a control mechanism determining the position of the valve member in response to an angular rotation of the rotor. The bracket includes inner and outer plates each provided with an aperture or apertures to permit air flow therethrough. The inner plate is provided with a first cylinder portion to support therein a second cylinder portion the valve member non-rotatably and axially displaceable. The rotor is rotatably supported on the outer plate of the bracket in the form of a cylinder loosely fitting in the second cylinder portion of the valve member.