Membranes and abdominal aortic grafts made of Polyurethane VC and of polyurethane foam were inserted in 35 dogs. The reaction of the tissues to polyurethane was studied and the functioning of the aortic grafts with smooth internal surfaces (membrane form of polyurethane) was compared to that of those with spongy internal surfaces (foam form of polyurethane). Polyurethane VC membrane became encapsulated by an envelope of fibrous tissue and appeared to lose some strength inside the body, but fibrous tissue grew into polyurethane foam, strengthening it. The nine smooth grafts made of Polyurethane VC membrane all thrombosed, but 80% of the foam grafts remained open. This confirmed the observations of other workers that a smooth internal surface is undesirable in arterial transplants. The grafts of polyurethane foam functioned well as aortic replacements. They have many of the requirements of an ideal arterial transplant and are being studied further. Submitted on April 1, 1959