When used as a valve bioprosthesis, bovine pericardium must shear internally to bend smoothly during the cardiac cycle. Twelve fresh and 12 glutaraldehyde-fixed bovine pericardial tissue samples were mounted in a shear testing apparatus immersed in a 20°C bath, and sheared circumferentially and radially at strain rates of 1.0, 0.1 and 0.02 s^-1. Six fresh and six fixed samples were also tested at 37°C. Stress relaxation over 100 s was measured for each shear rate. In contrast to fresh pericardium, which sheared easily at low shear stress (shear modulus 1.0 kPa) and increased to a modulus of 5 kPa at a shear strain of ∼1.0, glutaraldehyde-fixed pericardium immediately resisted shear stress, reaching a shear modulus of 15-20 kPa at 0.2 strain. No orientation effects were observed, and shear relaxation was similar for fresh and fixed tissue. Marked tissue stiffening after glutaraldehyde fixation may be a factor in bioprosthetic heart valve failure.