In order to obtain better insight into the changes in material properties of aortic valve tissue due to the treatment with glutaraldehyde, comparative tensile and relaxation experiments have been performed with strips taken from porcine aortic valve tissue in a fresh condition and after the treatment. To eliminate biological deviations between different strips, the comparative experiments were done on the same strip. The usual treatment for making a bioprosthesis, where an entire valve root is preserved under hydrostatic pressure of 13 kPa, was simulated by treating the strips under a uniaxial preload of 0.3 N. It was expected that the magnitude of the hydrostatic pressure over the valve during the treatment affects the material properties of the treated tissue. So, the treatments under hydrostatic pressures of 0 and 0.5 kPa were also simulated, by treating the tissue strips with uniaxial preloads of 0 and 0.01 N respectively. As a check leaflet strips of a Hancock bioprosthesis were also measured. To describe the elastic behaviour, two Young's moduli are defined: one in the low strain region (E0) and one in the high strain region (E13). For describing the viscoelastic behaviour, use was made of the model of Fung (1972, Biomechanics, its Foundations and Objectives; Fung, Perrone and Anliker, Prentice Hall.) to fit the results. Here, the viscoelastic parameters to determine, were K, ¿1 and ¿2. Due to the treatment the stress-strain curve shifted to the stress-axis for the strips treated under 0.3 N and in the opposite direction for the strips treated under 0 and 0.01 N. For all strips after treatment the total amount of relaxation decreased, being the most pronounced (60%) for the strips treated under 0.3 N. For the tissue treated with preload of 0.3 N the changes in the elastic and viscoelastic parameters E0, E13 and K were the most pronounced: E0 changed from 0.18 × 106 to 1.7 × 106 N m-2, E13 from 6.6 × 106 to 23.0 × 106 N m-2, K from 0.085 to 0.024. On the contrary, the variation in the time constants ¿1 and ¿2 (from 0.0054 to 0.0034 s and from 71 to 37 s respectively) was hardly significant. The elastic behaviour of the Hancock leaflet strips confirms that of the tissue strips treated under 0.3 N, while the viscoelastic behaviour differs somewhat.