Static tensile test, stress relaxation test and fluid dynamical test were carried out to investigate the relatioships between the mechanical deformation or stiffness of leaflets and the opening behaviour of bioprosthetic valves. The specimens used were fresh and glutaraldehyde (GA)-treated canine aortic valves. The deformability was found in the fresh specimen, medium in the 0.05% GA-treated tissue, while the 0.1-5.0% GA-treated tissues had the minimum deformability with almost the same material properties. In the fluid dynamical tests, the opening resistances of fresh and 0.05% GA-treated valves were less than those of 0.1-5.0% GA-treated valves. It was elucidated that the valve functions were closely related to the material properties of aortic valve leaflets.