Abstract: A power steering mechanism for a vehicle which comprises a rotary steering control member and a driving piston connected to a member in the steering gear of the vehicle. The piston separates two pressure chambers in the housing. The mechanism also comprises a shaft of which the first end defines a screw cooperating with a nut attached to the piston. A pressure fluid distribution valve is adapted to control fluid communication between the pressure source formed by a pump, and the two pressure chambers. The distribution valve is of the rotary type and comprises a sleeve moving as a whole with the rotary control member and a rotor moving in unison with the shaft. The shaft and the rotor are connected to opposite ends of a torsion bar. The second end of the shaft is broadened and contains a bore. This other end is provided with orifices and defines the sleeve of the rotary valve, in which the rotor is mounted for rotation.