A lead for an implantable cardiac prosthesis having an integrated protection against the effects of magnetic resonance imaging (“MRI”) fields. A protection circuit (26) may be placed at the distal end of the lead comprises a resistive component (28) interposed between the electrode (E1, E2) and the distal end of the conductor (22, 24) associated with this electrode. A normally-open controlled active switch (34, 36) may allow in its closed state to short-circuit the resistive component. A control stage (32) may be coupled to the conductors and detect the voltage of a stimulation pulse applied on the conductor(s), and selectively control by this voltage the closing of the active switch for a duration at least equal to the duration of detected stimulation pulse.