An endocardial lead system for stimulation of the left ventricle, adapted to be combined with a generator of an active implantable medical device for the delivery of cardiac stimulation and/or resynchronisation and/or defibrillation pulses, this system including: - a lead adapted to be implanted in the tissues of an inner region of the myocardium, by penetration in the thickness of the wall of the interventricular septum or in the thickness of the free wall of the left ventricle, under the surface and along this wall between the apical region and the atrial region, this lead (34, 34') being a micro-lead consisted, in its distal active part, by a micro-cable (36, 36') including an electrically conductive core coated with an electrically insulating layer, the active part comprising a series of naked zones (38, 38') of the micro-cable, forming stimulation electrodes that are electrically connected together; and - an accessory for implantation of the lead, characterized in that: - the diameter of the micro-cable (36, 36') is at most of 1 French (0.33 mm); - the free distal end of the micro-cable is an end (50, 50') that is folded over itself; - the implantation accessory includes a needle with a free acute puncturing end and an opposite end mounted on a gripping tip, this needle being, at least in its distal part, a hollow needle including an inner through-lumen; - the folded part of the distal end of the micro-cable is inserted in the inner lumen of the needle, and the non-folded part of the micro-cable extends against the outer surface of the needle, along the latter, up to the gripping tip; and - the implantation accessory further includes releasable means for maintaining the micro-cable, for the support and the holding of the micro-cable over the length of the needle between the puncturing end and the gripping tip, so as to allow the implantation of the micro-lead by simultaneous penetration of the needle and of the micro-cable carried by this needle in said thickness of the wall of the interventricular septum or of the free wall of the left ventricle. - a lead adapted to be implanted in the tissues of an inner region of the myocardium, by penetration in the thickness of the wall of the interventricular septum or in the thickness of the free wall of the left ventricle, under the surface and along this wall between the apical region and the atrial region, this lead (34, 34') being a micro-lead consisted, in its distal active part, by a micro-cable (36, 36') including an electrically conductive core coated with an electrically insulating layer, the active part comprising a series of naked zones (38, 38') of the micro-cable, forming stimulation electrodes that are electrically connected together; and - an accessory for implantation of the lead, characterized in that: - the diameter of the micro-cable (36, 36') is at most of 1 French (0.33 mm); - the free distal end of the micro-cable is an end (50, 50') that is folded over itself; - the implantation accessory includes a needle with a free acute puncturing end and an opposite end mounted on a gripping tip, this needle being, at least in its distal part, a hollow needle including an inner through-lumen; - the folded part of the distal end of the micro-cable is inserted in the inner lumen of the needle, and the non-folded part of the micro-cable extends against the outer surface of the needle, along the latter, up to the gripping tip; and - the implantation accessory further includes releasable means for maintaining the micro-cable, for the support and the holding of the micro-cable over the length of the needle between the puncturing end and the gripping tip, so as to allow the implantation of the micro-lead by simultaneous penetration of the needle and of the micro-cable carried by this needle in said thickness of the wall of the interventricular septum or of the free wall of the left ventricle.