Abstract: Methods and systems are provided for drilling a well bore 60 through a subterranean formation using a drilling rig 25 and a drill string 50, whereby the bottom hole pressure while circulating drilling fluid (“ECD”) may be substantially maintained when circulation is interrupted or altered, such as when adding a joint of drill pipe to or removing a joint of drill pipe from the drill string. The method includes controllably applying and maintaining a desired variable annulus fluid pressure in the well bore, and thereafter controllably releasing the pressure from the well bore 60. In addition, methods and systems are provided for rotating the drill string while trapping, maintaining and/or releasing the well bore pressure. A substantially constant ECD pressure may be maintained on a formation, thereby facilitating the use of a lower density drilling fluid than may otherwise be required to maintain well control.