Abstract: An implantable electrical device that includes a control unit and one or more leads. The leads are configured to be implanted in the chambers of the heart of a patient and the control unit is configured to be implanted in the body of the patient. The control unit induces the leads to provide therapeutic electrical stimuli to the heart. The control unit also includes a magnetoresistive sensor that senses the presence of an externally applied magnetic field. The sensor outputs a signal which has a value that is proportionate to the sensed magnitude of the externally applied magnetic field. The control unit initiates a preprogrammed activity, which for example can be either a function or a change in the mode of operation of the device, when it receives a signal from the magnetic field sensor indicating that the magnitude of the sensed magnetic field has a value corresponding to the magnitude of a magnetic field that occurs as a result of positioning a selected external magnet adjacent the control unit.