An assembly, implantable in the coronary venous network, for the stimulation of a left heart chamber by means of a generator of an implantable active medical device, in particular a cardiac stimulator or resynchroniser, said assembly comprising, in combination: - a micro-cable made of a conductive material, comprising at its proximal part means for its coupling to the generator of the implantable active medical device; and - a catheter made of an insulating material, comprising a hollow tube housing the micro-cable in an internal lumen with possibility of relative axial translation between the catheter and the micro-cable, the assembly being characterized in that: - the catheter is an implantable micro-catheter of diameter at most equal to 2 French (0.66 mm), made of biocompatible material(s) adapted for a permanent implantation in venous networks; - at its distal part, the micro-catheter comprises at least one lateral window comprising a through-orifice formed in the wall of the hollow tube, and adapted to put opposite to each other, with no galvanic obstacle, a region of the micro-cable surface located in said internal lumen in alignment with this window with a stimulation site defined on the wall of the target-vein opposite the micro-catheter window, said region forming a stimulation electrode; - at its distal part, the micro-cable is a non-insulated micro-cable at least in the region of the micro-catheter window; and - the micro-catheter is adapted to be telescopically displaced on the micro-cable in said distal part between an extended position, in which the respective distal ends of the micro-cable and the micro-catheter are closer to each other, and a retracted position, in which said respective distal ends are far away from each other, so as to modify the position of said site of stimulation of the target-vein by displacement of the micro-catheter on the micro-cable. - a micro-cable made of a conductive material, comprising at its proximal part means for its coupling to the generator of the implantable active medical device; and - a catheter made of an insulating material, comprising a hollow tube housing the micro-cable in an internal lumen with possibility of relative axial translation between the catheter and the micro-cable, the assembly being characterized in that: - the catheter is an implantable micro-catheter of diameter at most equal to 2 French (0.66 mm), made of biocompatible material(s) adapted for a permanent implantation in venous networks; - at its distal part, the micro-catheter comprises at least one lateral window comprising a through-orifice formed in the wall of the hollow tube, and adapted to put opposite to each other, with no galvanic obstacle, a region of the micro-cable surface located in said internal lumen in alignment with this window with a stimulation site defined on the wall of the target-vein opposite the micro-catheter window, said region forming a stimulation electrode; - at its distal part, the micro-cable is a non-insulated micro-cable at least in the region of the micro-catheter window; and - the micro-catheter is adapted to be telescopically displaced on the micro-cable in said distal part between an extended position, in which the respective distal ends of the micro-cable and the micro-catheter are closer to each other, and a retracted position, in which said respective distal ends are far away from each other, so as to modify the position of said site of stimulation of the target-vein by displacement of the micro-catheter on the micro-cable.