The development of postoperative pericardial adhesions increases the risk of cardiac reoperations because of the danger of damaging the heart, great vessels, or grafts. Several pericardial substitutes have been tested in the past in an attempt to facilitate reoperation, with inconclusive results. This study evaluated eight different materials as pericardial substitutes: six synthetic materials and two different preparations of bovine pericardium. In 32 dogs a 10 by 5 cm piece of pericardium was excised through a right thoracotomy and the defect closed with a measured patch. Each material tested was implanted in four dogs that were put to death at 3, 6, 9, and 12 months. At autopsy the development of adhesions and epicardial reaction were graded as none, minimal, moderate, and severe. Histologic studies of the patch, the epicardium, and the suture line were performed. Our results suggest that both types of bovine pericardium were an excellent substitute. Although minimal adhesions developed, these were easily dissected. The underlying anatomy was clearly recognizable because of the lack of epicardial reaction. Silicone rubber-coated polyester fabric was an acceptable material for the prevention of adhesions, but a severe fibrous epicardial reaction impeded the recognition of the coronary arteries. Both silicone-filled and high-porosity polytetrafluoroethylene (PTFE) films reduced adhesions but caused a severe epicardial reaction. The other synthetic materials were considered inferior because of severe epicardial reaction and/or structural deterioration.