Abstract: Special circulation subs mostly consist of an outer barrel which is provided in a cross-sectional plane with three circulation ports, which ports are closed during normal operation of the sub by means of a movable sleeve. The movement of such a sleeve is operated by steel or hard rubber balls which are pumped down the drill string.In many cases it is desirable to be able not only to open the sub, but also to close it again. This is achieved by providing a spring loaded inner sleeve. That inner sleeve has a groove at its outer periphery to receive a plurality, preferably three balls which are adapted to cooperate via openings in the sleeve with two grooves in the outer barrel. Closing of the sub can be caused by a return sleeve, also controlled by a plurality of balls and a spring, located underneath the sleeve.A special embodiment is characterized by a helical groove in the inner surface of a port sleeve.