A kit for penetrating the cardiac septum and for the placement of a trans-septal device, in particular of a stimulation probe, in a left heart cavity. This kit includes a penetration guide (16), a handling stylet, a piercing stylet (38) and a guiding stylet (46). The penetration guide (16) comprises a probe body with a flexible hollow sheath (22) successively receiving these stylets, and a probe-head (18, 24) having an extendable helicoid screw (20) for anchoring to the wall of the septum (10). The piercing stylet (38) presents a sharp distal extremity (44) emerging from the frontal face, through the anchor (20), over a distance at least equal to the local thickness of the septum (10), so as to traverse through the septum. A guiding stylet will be then introduced into the sheath to form, after withdrawal of the penetration guide (16), an axial guide traversing the septum, for the introduction therethrough of a trans-septal device.