Twenty-five canine aortic valves were sterilized and preserved for 2 to 7 days. Fifteen of these valves were implanted, as allografts, in the mitral position. Valve leaflet architecture and cell viability were assessed at regular intervals by means of histochemical, tissue culture, and scanning electron microscopic techniques. Results suggested that fresh canine allograft valves undergo progressive changes in leaflet architecture and fibrocyte viability. The predominant structural changes involve the loss of leaflet endothelium and disorganization of underlying collagen fibers. These changes are best appreciated with the scanning electron microscope and may help to explain the gradual degeneration of allograft valve transplants in man.