1. Preliminary studies of polyurethane resins of the reactive prepolymer type indicate that they may be of considerable value in experimental and clinical cardiovascular surgery as adhesive agents. 2. These resins combine hardness, elastomeric (rubber-like) properties, and chemical inertness which persist unchanged for periods of at least six months. 3. The position and quantity of isocyanate groups in the prepolymer molecule, together with the molecular weights of the prepolymer and polyol, determine the physical properties of the final polymer obtained. 4. Exceptional adhesion between the polymer and blood vessel wall is a result of the reaction between available hydrogen atoms in the substrate tissue and the isocyanate groups in the prepolymer.