Aortic valves were transplanted from sheep and pigs into the descending thoracic aortas of nine groups of dogs. The valves were fresh, freeze-dried, or treated with propiolactone. Half the groups received azathioprine. The following conditions led to the best results: (1) transplantation of a minimum of aortic wall with the valve; (2) good initial alignment and movement of the cusps; (3) administration of azathioprine for eight weeks; and (4) support of the graft by a Dacron tube, preventing rupture and separating the graft from ingrowing host tissue.