In a series of 23 dogs, glutaraldehyde-fixed xenograft pericardium was implanted as a left atrial wall patch (Group I, 11 dogs), as a substitute for the mitral valve chordae (Group II, 12 dogs), and as an external pericardial patch (Group III, five dogs, in which one of the two former procedures was associated). All dogs of Group I survived and were put to death after 12 months. The atrial patch appeared thickened and shrunken in all, with intense fibrous reaction, calcification, and even bone formation. The structural changes were minimal in the external aspect of the patch, not in contact with the host's endocardium and blood. Eight dogs of Group II survived from 7 months to 3 years. In all, the pericardial chordae were thickened and calcified with good healing at the suture lines; that their original length was retained, however, allowed for good valve function. In the dogs of Group III the pericardial substitute appeared well preserved after 12 months, not calcified, and without noteworthy degenerative changes. Minimal or absent epicardial adhesions were noted. We conclude that (1) the tendency for xenograft pericardial tissue to calcify is strongly influenced by the site of implantation and function and by contact with host blood and endocardium; (2) heterologous pericardium may be considered suitable for closure of pericardial defects after open cardiac operations, and (3) studies on calcification of biological tissue should be carried out by long-term evaluation of such tissues inside the heart.