A method for maintaining pressure in a wellbore during drilling operations includes the steps of providing fluid from a reservoir through a drill string, circulating the fluid from the drill string to an annulus between the drill string and the wellbore, isolating pressure in the annulus, measuring pressure in the annulus, calculating a set point backpressure, applying back pressure to the annulus based on the set point back pressure, diverting fluid from the annulus to a controllable choke, controllably bleeding off pressurized fluid from the annulus, separating solids from the fluid, and directing the fluid back to the reservoir. An apparatus for maintaining pressure in a wellbore during drilling operations includes an adjustable choke for controllably bleeding off pressurized fluid from the wellbore annulus, a backpressure pump for applying a calculated set point backpressure, and a processor for controlling the adjustable choke and backpressure pump.