An implantable medical electrode device, in particular a cardiovascular cardiac pacemaker or defibrillator electrode device, comprises an elongate, tubular electrode body (2), a fixing zone (4) in front of the distal end (3) of the electrode body (2), in which the externally closed peripheral envelope (8) of the electrode body (2) is reversibly expandable into a body lumen (27) for detachable fixing of the electrode device (1), and an expansion apparatus, situated in the fixing zone (4), for controlling the expansion and contraction in the fixing zone (4).