A long-term experimental morphological study was carried out in 22 adult sheep to evaluate a new pericardial valve prosthesis (Pericarbon), which had been implanted in the tricuspid or mitral position. This prosthetic device differs substantially from others in that its construction design consists of two sheets of glutaraldehyde-fixed bovine pericardium and a low-profile flexible plastic stent (Delrin) covered by a pyrolytic carbon coated dacron fabric; one pericardium sheet forms the three cusp valve and is sutured to the second, which lines the inner surface of the plastic stent. Twenty animals were sacrificed at fixed intervals, while 2 are still living at about 3 years post-surgery. Tricuspid explants (mean duration, 295 days) showed significant fibrous sheathing and a mean calcium X-ray score of 1.75. Mitral medium-term explants (mean duration, 325 days) had fairly well preserved pliability and a mean calcium X-ray score of 2.5. Long-term explants (mean duration, 467 days) were all stiffened by calcification (mean score, 3.75). None of the explants had tears or perforations. Medium or long-term mechanical failure was not observed. A significant host tissue reaction took place in the tricuspid but not in the mitral position. Calcification mainly involved the collagen fibres and increased progressively with time. Ultrastructural studies invariably disclosed fair preservation of graft tissue structures, surface reendothelization and initial nuclei of calcification within the collagen fibres. These morphological findings confirm the potential advantages of this new prosthetic device and warrant long-term clinical trials to test its actual durability.