Pulmonary valve autografts and allografts have been recently reported as being clinically effective for the replacement of the diseased aortic valve. While the biomechanics of the aortic valve have been widely studied, there is little information available about the mechanical properties of the pulmonary valve. We felt that it was necessary to investigate the mechanical properties of the pulmonary valve to determine if it is mechanically suitable as a long term replacement for the aortic valve. We employed physiological strain rate tensile testing to investigate the stress-strain and stress relaxation behaviour of 44 aortic and 40 pulmonary valve cusp strips cut in the radial and circumferential directions, in fresh and glutaraldehyde fixed states. Stress-strain and stress relaxation tests were performed on each test strip at extension rates of 40 mm/s, 4 mm/s and 0.4 mm/s. In all but one mechanical parameter, we found no difference between aortic and pulmonary valve tissues. The extensibilities and relaxation rates were similar, but the aortic valve tissue had a greater average modulus (p = 0.0005) than the pulmonary valve tissue (i.e. 7.41 MPa vs. 5.86 MPa respectively). Since bioprostheses are often constructed from materials with mechanical properties very different from those of the aortic valve, like pericardium, the slight difference between aortic and pulmonary valves is unlikely to affect the operation of the pulmonary valve in the aortic position. The pulmonary valve could therefore be considered mechanically acceptable as a replacement for the aortic valve.