Abstract: A mechanically automatically operated volumetric flow regulator for interior ventilation systems through which a medium flows has a housing that comprises a wall and forms part of a flow conduit. A flap valve is mounted in the interior of the housing on a shaft arranged transversely to the flow direction. The flap valve is swingable against between open and closed positions. In order to describe a volumetric flow regulator in which the flap valve position is identifiable even from outside the housing without disassembly, an indicating element, indirectly or directly connected to the flap valve and/or to the shaft and likewise moving in accordance with the movement of the flap valve, is to be provided inside and/or outside the housing, said element being arranged, for visual monitoring of the position of the flap valve, in a manner freely visible from outside the housing.