Abstract: A modulated bias unit is provided for controlling the direction of drilling of a rotary drill bit when drilling boreholes in subsurface formations. The unit comprises a plurality of hydraulic actuators spaced apart around the periphery of the unit and having movable thrust members hydraulically displaceable outwardly for engagement with the formation of the borehole being drilled. Each actuator has an inlet passage for connection to a source of drilling fluid under pressure and an outlet passage for communication with the annulus. A selector control valve connects the inlet passages in succession to the source of fluid under pressure, as the unit rotates, and a choke is provided to create a pressure drop between the source of fluid under pressure and the selector valve. A further choke is provided in the outlet passage from each actuator unit. The actuators and control valve arrangements may take a number of different forms.