Porcine bioprosthetic heart valves (PBHVs) have excellent short-term durability, but have generally poor long-term performance. Although the exact mechanism is still unknown, it is almost certain that mechanical factors play a significant role in PBHV failure. Aortic valve cusps are highly anisotropic with a complex, non-uniform structure (Fig. 1, top). It is therefore understandable that to date there have been no quantitative studies to determine how mechanical behavior is altered by mechanical fatigue. In this study we performed biaxial mechanical tests and uniaxial failure tests on cusps harvested from 4mmHg glutaraldehyde treated PBHVs loaded intact up to 300 × 106 cycles.