Abstract: The device permits dosing a secondary fluid in a principal fluid and comprises a body (1) in which is provided a passageway (20) for the circulation of the principal fluid and having an inlet orifice (22) and distribution orifices (23) for the fluid, which distribution orifices open onto a cylindrical chamber (3) defined by a housing (2), the chamber (3) containing means (39, 16, 19, 27, 28, 30) for driving a pump (36) of the secondary fluid which comprise three pistons (30) mounted on radial rods (16) pivotally mounted on an arm (15) of a crank (39) adapted to drive in alternating translation a dosing piston (35) of the pump (36). Each piston (30) is slidable in a cylinder (28) pivotally mounted on a pin (40). This measuring-dosing device delivers additive in proportion to the rate of flow of the principal fluid in synchronism with the crank (39) and without the use of any spring.