Abstract: A headbox for a papermaking machine or the like in which material, such as supension, flows from the headbox between upper and lower lips and through a discharge slot defined by the lips to a wire or a like receiver. The upper lip is reinforced by a beam extending along the upper lip on top of the lip while a detector is provided for detecting deflection of the upper lip due to the forces exerted thereon by material flowing between the lips. The detector is arranged to emit signals in conformity with deflection of the upper lip and correcting influences can be initiated in conformity with the emitted signals to compensate for the aforementioned deflection.