An implantable electrical medical lead for a pulse generator includes a distal electrode, a connector for electrical connection with the pulse generator, an electrical conductor within the lead extending between the connector and the distal electrode, and an insulating sheath of flexible resilient material normally covering the electrical conductor having self sealing properties to re-insulate the electrical conductor in the event a rupture of the sheath should occur which exposes the electrical conductor. The insulating sheath is of polymeric material embedded with a plurality of pellets, each pellet having a frangible shell containing a liquid sealing material that solidifies when the frangible shell is broken and thereby filling in any voids in the polymeric material.