Collagen and elastin fibres are of major importance in providing the aorta with tensile strength and elasticity. The presence of cross-links in collagen and elastin is essential for the mechanical stability of collagen and elastin fibres. beta-aminopropionitrile (BAPN) reduces the formation of cross-links by inhibiting the enzyme lysyloxidase. Young rats were injected with BAPN to inhibit the formation of cross-links, and the changes in the biomechanical and biochemical properties of the thoracic aorta were studied. The biomechanical analyses of aortic samples from BAPN-treated rats showed a significantly increased diameter (1.64 +/-0.02 mm), a significantly reduced maximum load (1.08+/-0.08 N), and a significantly reduced maximum stiffness (3.34+/-0.10 N) compared with controls (1.57+/-0.02 mm, 1.55+/-0.04 N and 4.49 +/-0.14 N, respectively). No changes in the concentrations of collagen and elastin were found. The content of pyridinoline, a mature collagen cross-link, was significantly decreased by 49% in the BAPN-treated group compared with controls. No changes in the concentration of desmosine + isodesmosine, the major cross-links of elastin. were found. The present study shows that cross-links are essential in providing mechanical stability of the aorta. Even a partial inhibition of the cross-linking processes results in a destabilisation of the aortic wall with increased diameter and reduced strength and stiffness.