The present invention relates to acirculation subassembly (circsub) for incorporation in a drill pipe. The circsub has a flow through condition in which the circsub allows fluid to flow through a bore of the circsub and does not allow fluid communication between the bore of the circsub and an annulus located outside the drill pipe, a partial bypass condition in which the circsub allows fluid to flow through the bore of the circsub and allows fluid communication between the bore of the circsub and the annulus, and a full bypass condition in which the circsub does not allow fluid to flow through the bore of the circsub and allows fluid communication between the bore of the circsub and the annulus. An actuator is provided to change the circsub between conditions under the control of a controller.