The manner in which the pericardium adapts to chronic cardiac dilation is not known. Recent work from our laboratory indirectly suggested that the size of the pericardium and/or its pressure-volume relation was altered by chronic cardiac enlargement. To examine this question further, we compared pericardial pressure-volume and stress-strain relations, surface area, mass, and average thickness in seven normal dogs and seven with chronic volume overload hypertrophy due to a systemic arteriovenous fistula. Dogs with significant cardiac hypertrophy had an increased pericardial volume at any pressure and a proportionality constant for the slope of the entire curve as determined by nonlinear regression analysis. This was associated with parallel increases in pericardial surface area and mass such that average thickness was unchanged. Stress-strain analysis of the data revealed that the mechanical properties of the pericardium were not significantly different in dogs with chronic cardiac dilation. These results indicate that during chronic cardiac dilation the pericardium enlarges in size and mass. The pericardial chamber is more compliant, although the intrinsic stiffness of the pericardium appears to be unchanged. Further, since pericardial mass is increased, the response to chronic dilation appears to also involve the addition of new pericardial tissue.