Arterial stiffness describes the rigidity of the arterial wall. In the last decade, there has been increasing interest in the potential role of arterial stiffening in the development of cardiovascular disease in adults. Arterial stiffness is primarily determined by structural components of the arterial wall, elastin and collagen in particular, vascular smooth muscle tone, and transmural distending pressure.168 Increasing evidence suggests a role for endothelium in the regulation of arterial stiffness through the influence of smooth muscle tone by release of vasoactive mediators. Indeed, the influence of basal nitric oxide production and endothelin-1 on stiffness of the common iliac artery in an ovine model have recently been shown. Additionally, it has been shown that atrial natriuretic peptide and, to a lesser extent, brain natriuretic peptide can modify iliac artery stiffness in this animal model. The significance of arterial stiffness, as alluded to earlier, owes to its direct relationship with characteristic impedance, hence the pulsatile component of the arterial afterload, and its effect on the timing of return of the reflected waves from peripheral sites.