Abstract: A rotary actuator is disclosed which changes the angular position of the rotary valve which is used to determine the attenuation presented by a shock absorber. The rotary actuator is disposed within a cylindrical piston rod of the shock absorber, and comprises a power transmission mechanism for driving the rotary valve for rotation between a first and a second position within a given range, a d.c. motor for driving the mechanism, a first stop mechanism for selectively stopping the rotation of the motor at a third position which is situated centrally within the given range, and a solenoid assembly for making the first stop mechanism operative. These components are disposed within the cylindrical piston rod in the sequence named from below to above. The power transmission mechanism includes a second stop mechanism which prevents the rotary valve from rotating outside a first and a second location which determine the given range.