A ready-to-install electrode system designed for use with an endoscope for performing cardioversion through the esophagus, and an assembly of the system and an endoscope. The system is characterized in that it includes a globally tubular protective cover (3) designed to be fitted on the endoscope (40) to cover at least the distal end of the endoscope, at least one electrode (2a-d) including a conductive electric membrane (21) connected to an electric conduction wire designed to be connected by its free end to a cardioversion apparatus, first and second fixing elements adapted to fix the at least one electrode respectively on the distal end of the endoscope and on the tubular wall (31) of the cover, such that the conductive membrane is directly accessible from outside the cover.