The mechanical behaviour of canine, human and bovine isolated pericardia was investigated. The three types of tissue exhibited the same basic mechanical response, following a similar J-shaped stress-strain curve, the canine pericardium being the stiffest and strongest one. Our findings on bovine pericardium showed no strain rate dependence, the tissue being essentially isotropic. The selective digestion of the collagen fibres by collagenase was used to assess, separately, the mechanical contribution of the main pericardial components, the collagen and elastin fibres. Our results indicate that the initial low modulus (E1) is only slightly affected by coilagenase, whereas the high E2 modulus of the tissue is dramatically affected, decreasing to less than 10% of its initial value. In light of ourfindings, it is maintained that collagen fibres are responsible forthe mechanical response of the tissue under high stresses, whereas the initial low modulus reflects elastin fibre's contribution.