Recent investigations show that glycosaminoglycans (GAGs) and proteoglycans (PGs) have the ability to affect lipid peroxidation, one the best characterized forms of free radical mediated biological damage. A protective effect of these extracellular matrix (ECM) components has been demonstrated in various experimental systems, including fatty acids and liposomes, where oxidation was induced by transition metals, including copper and iron. The effect was specific and dependent on the type and structural features of GAGs and PGs. The mechanism of peroxidation inhibition was likely to be dependent, at least to a large extent, on the sequestration of transition metals by GAG chains. Thus, it is conceivable that GAGs in the ECM and in the pericellular space may contribute to protecting cells against free radical damage. It is of particular interest that in certain tissues (cornea and aorta) aging was associated with a decrease of content of the GAGs which were most effective as anti-oxidant. This suggests that age-induced modifications of ECM composition in certain tissues may increase the susceptibility to oxidative stress. The investigation on the effect of GAGs on lipoprotein oxidation led to apparently conflicting results. An interesting reconciliation is possible, according to which GAGs exerted their protective effect under experimental conditions not compatible with the formation of lipoprotein-GAG complexes; rather, lipoproteins exhibited increased susceptibility to metal-catalyzed oxidation (MCO), possibly due to structural modifications of the particle after binding to GAGs or PGs. This process is likely to occur in the intimal matrix of arteries.