An adult-size heterograft aortic valve patch sterilized with β-propiolactone or radiation, consisting of the aortic wall above and the anterior mitral valve leaflet below and bearing one aortic leaflet, was used to widen the right ventricular outflow tract and pulmonary artery in 12 puppies. Except for one animal killed at 12 days, pulmonary valve insufficiency was present in all animals. A calcified thrombus filled the sinus of Valsalva in all but one of the leaflets; the graft leaflets and patches were thickened and contracted. A fibrous intimal sheath produced a shelf-like ridge at the pulmonic valve area. Focal areas of mineralization were seen in this sheath as well as in the thrombus. The changes were somewhat less marked in the irradiated grafts than grafts sterilized with β-propiolactone. Despite the presence of graft regurgitation, unexcised host pulmonary cusps grew to normal size as the recipients grew.