Abstract: A power assisted steering system for a vehicle including a steering wheel operatively connected to steered wheels comprising a housing, a nut-and-screw mechanism comprising a nut member and a screw member, an input shaft mounted in said housing, at least one flexible shaft connected at one end to the steering wheel and at its opposite end to said input shaft, intermeshing gear means carried by said input shaft and one of said members of said nut-and-screw mechanism, means operatively connecting the other member of said nut-and-screw mechanism to the steerable wheels, the members of said nut-and-screw mechanism being of a configuration to preclude rotation relative to one another whereby upon rotation of one of said members, the other member is actuated in a rectilinear direction to thereby effect movement of the steered wheels, hydraulic circuit means including a double-acting servo-cylinder and a control valve including a slide member for controlling flow of hydraulic medium to and from said servo-cylinder,