The guide wire (207) has an arrangement for reversibly locking its end at or near the proximal end of the electrode line (201). Deformation is used to increase the effective contact diameter of the guide wire to a value essentially corresponding to the inner diameter of the electrode line or to a greater value. Independent claims are also included for the following: (1) an implantable electrode line (2) an arrangement of a guide wire and an implantable electrode line.