Abstract: A cannula assembly suitable for rapid, closed-chest cannulation of the cardiopulmonary system to an extracorporeal bypass machine through punctures in the femoral veins and arteries is disclosed. The assembly comprises a conduit made of a smooth, flexible, thin-walled material and capable of longitudinal insertion through these vessels by use of an associated "introducer" and conventional guide wire. The conduit has adequate hemodynamic flow capacity and sufficient unsupported hoop strength to resist collapse from suctional and vasoconstrictive forces during use.