Age-related arterial stiffening plays a major role in the 235 000 deaths caused by heart disease in the UK annually. Fragmentation of elastin, a compliant and resilient protein found in the arterial wall, may be implicated in its pathogenesis. Elastin rings, obtained from pig arteries, were subjected to fatigue testing, in which ring specimens were cyclically stretched and the number of cycles to failure at different mean extensions was measured. Inflation tests on whole arterial segments maintained at their in vivo length were also performed on a specially-designed fatigue-testing machine. Another facet of the project involved examining the effect of increasing arterial strain on the interlamellar spacing in pig carotid artery so that variations in interlamellar spacing and radial strain distribution across the arterial wall could be determined. Finally, an anomalous region was seen in pig carotid artery, where an axial rather than circumferential medial layer, existed outside the 'normal' media. Other mammals were examined, and mechanical tests on both layers were performed, having separated them by a freezing and machining technique. Fatigue testing of elastin rings showed a significant inverse linear relationship between maximum extension ratio and number of cycles to failure. There was also an increase in compliance with increasing cycle number within each test. The results suggest that elastin undergoes fatigue fracture, at least in vitro, something that has so far only been hypothesised. Inflation fatigue tests were unsuccessful because of problems of stress concentrations at cannulation sites causing premature failure and leakage of the pressurising fluid. Lamellar spacing across the arterial wall increased significantly with increasing distance from the intima. This may be related to the increased stress that exists on the intimal side of an artery compared with adventitial side, or because of the anomalous layer that lies outside the 'normal' media. The outer medial section, initially observed in the pig, was also found in sheep, cow and horse carotid artery, but not in mouse, rat, cat and rabbit. Stress/strain measurements of the two separate layers showed that, in the circumferential direction, the outer media appeared to be more compliant than the inner media, and, in the axial direction, the inner media appeared to be slightly more compliant than the outer media. Arterial structure is complex and inhomogeneous. The distribution of local mechanical properties and their fatigue characteristics reflect this inhomogeneity. Further fatigue testing and modelling of arterial structure-function relationships will be required in order to advance the research described here.