An arrangement in the headbox of a paper or cardboard machine has a pulp suspension feed system distributing the suspension over the width of the headbox. At least one first flow resistance element, such as an aperture plate and/or tube system, is provided with flow apertures, from which headbox the pulp suspension is passed via at least one chamber, such as an equalizing chamber and/or the slice gap of a slice chamber, onto a forming wire or equivalent. The arrangement comprises at least one pulp flow guide plate, preferably arranged mainly in the transverse direction of the headbox, having at least one groove formed in its guide surface.
