A method and apparatus to regulate the down-hole hydrostatic pressure in a wellbore are provided which depend on regulating the resistance to the flow of wellbore returns produced by the wellbore. The resistance may be provided by the internal gas pressure in a gas/liquid separator receiving the flow of wellbore returns, where the internal gas pressure is regulated by an adjustable back pressure valve and a gas source. Alternatively or in addition, the resistance may be provided by a pump receiving the flow of wellbore returns, where the resistance of the pump is regulated by adjusting the speed of the pump.