The present invention provides methods and compositions relating to the use of late passage mesenchymal stem cells (MSCs) for treatment of cardiac rhythm disorders. The late passage MSCs of the invention may be used to provide biological pacemaker activity and/or provide a bypass bridge in the heart of a subject afflicted with a cardiac rhythm disorder. The biological pacemaker activity and/or bypass bridge may be provided to the subject either alone or in tandem with an electronic pacemaker.