After surgical removal of a portion of the cardiac wall, homologous dura mater cardiac grafts were sutured to the margins of the incision in the sternocostal wall of the right ventricle of the canine heart and recovered after 1, 4, 6 and 8 weeks of implantation. Representative tissue specimens were processed for and studied by means of SEM and TEM. The primary objectives were to study morphological changes in the dura mater grafts used to repair the lesions or defects in the cardiac wall and to compare alterations in the periosteal and arachnoidal aspects of the dura mater grafts after being implanted for varying periods of time. After one week of implantation, an amorphous layer of fibrin was deposited on or near the luminal surface of the original dura mater grafts. At four weeks of grafts implantation, a 'remodeling' process was apparent below the luminal surface of the graft. Although a large accumulation of fibrin was still present at this time, there was also an increase in the number of cellular and fibrillar components within the implant. Large numbers of macrophages and active fibroblasts were visible at this time along with new collagen. At the sixth week of implantation, an abundance of active fibroblasts, the presence of normal collagen and a darkly staining material interpreted as recently synthesized connective tissue components, fibrin deposits and/or degenerating collagen were also observed. Phagocytosis of the remaining fibrin was noted during this period indicating a continuation of the remodeling process at the luminal surface. Finally, after eight weeks of implantation, it was revealed that the original densely woven and relatively acellular graft had become infiltrated with various blood cells and vascular channels.