An implantable apparatus for controlling fluid flow within a host body includes a constricting member for allowing fluid flow within a body canal when in an open position, and for reducing fluid flow within a body canal when in an closed position. Remote telemetry can be used to operate the constricting member from a position outside the body. The constricting member can include a piston having at least two protrusions for contacting the vessel surface of the body canal. A fluid-operated member can receive fluid to reduce fluid flow within the body canal and expel fluid to allow fluid flow within the body canal. An actuating member is provided for operating the fluid-operated member between the open and closed positions. The actuating member has structure for flowing fluid into and out of the fluid-operated member. A control device is provided for operating the actuating member. An implantable drive mechanism for a constricting member or other medical device includes a sealed housing for implantation in a patient body, a drive magnet positioned within the housing, an actuator for moving the drive magnet, and a magnetically susceptible drive member positioned outside the housing such that movement of the drive magnet within the housing will move the drive member outside of the housing.