A generator for an implantable active medical device such as a cardiac prosthesis for stimulation, resynchronization and/or defribrillation, said device comprising: - a detection / stimulation lead including: • on the distal side, at least two electrodes (14, 16); • on the proximal side, a connector; and • at least two conductors (18, 20) extending over the length of the lead from the distal end to the proximal end thereof, to link the electrodes to respective terminals (24, 26) of the lead connector, and - said generator, adapted to be coupled to said lead, including: • a metal box; • an electronic detection/stimulation circuit (34, 36) housed in this box; • a connector comprising link terminals connected to the electronic detection / stimulation circuit and adapted to be coupled to respective terminals of the lead connector; and • MRI safety setting means, adapted to place the circuit of the generator in a configuration that is protected against the deleterious effects of an exposition of the lead to the magnetic fields liable to be encountered during an MRI magnetic resonance imaging examination, said generator being characterized in that the MRI safety setting means comprise means (SA, SV) for temporarily setting one of the link terminals (24, 26) of the connector to the potential of the metal box, the other link terminal(s) remaining connected to the electronic detection/stimulation circuit. - a detection / stimulation lead including: • on the distal side, at least two electrodes (14, 16); • on the proximal side, a connector; and • at least two conductors (18, 20) extending over the length of the lead from the distal end to the proximal end thereof, to link the electrodes to respective terminals (24, 26) of the lead connector, and • on the distal side, at least two electrodes (14, 16); • on the proximal side, a connector; and • at least two conductors (18, 20) extending over the length of the lead from the distal end to the proximal end thereof, to link the electrodes to respective terminals (24, 26) of the lead connector, and - said generator, adapted to be coupled to said lead, including: • a metal box; • an electronic detection/stimulation circuit (34, 36) housed in this box; • a connector comprising link terminals connected to the electronic detection / stimulation circuit and adapted to be coupled to respective terminals of the lead connector; and • MRI safety setting means, adapted to place the circuit of the generator in a configuration that is protected against the deleterious effects of an exposition of the lead to the magnetic fields liable to be encountered during an MRI magnetic resonance imaging examination, • a metal box; • an electronic detection/stimulation circuit (34, 36) housed in this box; • a connector comprising link terminals connected to the electronic detection / stimulation circuit and adapted to be coupled to respective terminals of the lead connector; and • MRI safety setting means, adapted to place the circuit of the generator in a configuration that is protected against the deleterious effects of an exposition of the lead to the magnetic fields liable to be encountered during an MRI magnetic resonance imaging examination,