In uniaxial tensile experiments in vitro mechanical properties of the different parts of porcine aortic valves, i.e. the leaflets, the sinus wall and the aortic wall, have been dealt with. Tissue strips cut in different directions were investigated. The collagen bundles in the leaflets show a stiffening effect and cause a marked anisotropy: within the physiological range of strains the largest slopes of the stress-strain curves of leaflet specimens in the bundle direction are a factor of about 20 larger than those of specimens taken along the perpendicular direction. For the sinus and aortic tissues, these values are 50–200 times smaller than those obtained from the leaflet specimens in the bundle direction. Two aspects of viscoelastic behaviour were examined: the strain rate sensitivity of the stress-strain curves and the relaxation behaviour. The stress-strain curves of the different valve parts appeared to be rather insensitive to the strain rate: the most pronounced sensitivity observed in our experiments, was a doubling of the stress at the same strain caused by a hundredfold increase of the strain rate. In analyzing the relaxation behaviour, use was made of the relaxation model proposed by Fung (1972, in Biomechanics, its Foundations and Objectives; Fung, Perrone and Anliker. Prentice Hall). In the leaflets, about 45% stress relaxation was found whereas this amounted to 30% in the sinus and aortic walls. Predictions based upon the model indicate that on cyclic loading the larger viscous losses have to be expected in the leaflets.