A system for the endocardial stimulation/defibrillation of the left ventricle. This system includes a generator (60) and an endocardial lead. The lead includes a lead body (26) whose distal end (30) extends into the right ventricle (14) and is provided with a mechanism to anchor (32) the distal end to the interventricular septum (20). The lead body carries on it a stimulating and/or defibrillation electrode (38) (64, 66). A microcable (42) extends into the lead body and beyond, with an intermediate portion (56) crossing from one side of the interventricular septum (20) to the other, and an active free portion (58) that emerges in the left ventricle (16). The microcable is coupled to the generator, to produce an electric field (62) between, on one hand, the stimulation electrode (38) or defibrillation electrode (64, 66) of the lead body and, on the other hand, a bare region of the active free portion (58) of microcable (42).