Histological, scanning and transmission electron microscope studies were made of normal human dura mater and cardiac valvular bioprostheses made of glycerol-treated human dura mater recovered after having been implanted in the aortic position (8 patients) or the mitral position (1 patient) for periods up to 4 years. Human dura mater has two layers: an inner or meningeal layer and an outer or endosteal layer. The surface of the inner layer is smoother than that of the outer layer. Both layers are composed mainly of large, wavy collagen fibrils (which are thought to correspond to type I collagen) and are relatively poor in elastic fibers and proteoglycans. Small calcific deposits were found in normal dura from older patients. Changes occurring in dura mater bioprostheses within 2 days after implantation consisted mainly of small surface thrombi. Calcific nodules, degenerated collagen and evidence of penetration of erythrocytes and plasma proteins into the cusps were observed in bioprostheses that had failed after being in place for 1-4 years. The calcific deposits and the degenerated collagen appeared structurally similar to those in glutaraldehyde-treated porcine aortic valvular bioprostheses. However, collagen fibrils in the latter were smaller than those in dura mater. Platelet aggregates on the cuspal surfaces were much less numerous in dura mater bioprostheses than in porcine aortic valvular bioprostheses. It is postulated that this difference is a function of the size of the collagen fibrils in the bioprostheses and that this accounts for the very low incidence of clinically evident thromboembolism in patients with implanted dura mater valves.