Scanning and transmission electron microscopic studies were made of: 1) 15 glutaraldehyde-treated porcine valvular heterografts that had been implanted in patients for periods of time ranging from 2 days to 76 months; 2) unimplanted, commercially processed porcine aortic valves; and 3) unprocessed porcine aortic valves. Unimplanted, commercially processed valves showed loss of endothelium and acid mucopolysaccharides. Short-term (< 2 months) post-implantation changes consisted of insudation of plasma proteins, penetration of erythrocytes into surface crevices, formation of a thin surface layer of fibrin, and deposition of macrophages, giant cells and a few platelets. Longer term (> 2 months) changes consisted of progressive disruption of collagen, erosion of valvular surfaces, formation of aggregates of platelets, and accumulation of lipid. The surfaces of the leaflets did not become covered with endothelium or with a fibrous sheath. It is concluded that progressive breakdown of collagen is a critical factor in determining the long-term durability of glutaraldehyde-treated porcine valvular heterografts.