Accurate control of bottomhole pressure (BHP) during drilling is essential as the industry meets increasingly challenging drilling environments such as narrow-drilling-margin formations, high-pressure/high-temperature (HP/HT) wells, and fractured formations. To increase the accuracy of BHP control during drilling, a fully automated prototype system consisting of a hydraulics simulator, a computer-controlled choke manifold, and a pump as part of the mud return system has been developed and tested. Rather than mud density alone, the system uses a variable backpressure at the annulus exit to achieve the required downhole pressure.