A method in the regulation of a pulp suspension flow through the headbox of a paper machine/board machine wherein an additional flow is introduced into the pulp suspension at different points across the width of the headbox. The concentration of the additional flow is regulated by a mixer unit which includes a displaceable distributor part such that when the mixing ratio is being regulated, the flow resistances of the component flows, of the additional flow, entering into the mixer unit are adjusted by displacing the distributor part of the mixer unit in a chamber of the mixer unit.