A body-implantable device for controlling the size of a fluid flow passageway within the body, typically for use in a Fenestrated Fontan operation. A housing is provided and secured within the body, typically to the artificial septum sutured into place in the Fontan operation. A flow passageway extends through the housing. An antenna is responsive to electromagnetic signals emitted from outside the body and conveyed through the body without wires to the antenna. Means are provided for varying the cross-section of the flow passageway in a manner responsive to signals received by the antenna.