Insoluble elastins were isolated from control and aneurytic aortas by a sequential extraction procedure involving the use of purified collagenase. Marked differences in amino acid analyses and susceptibilities to pancreatic elastase were observed between normal and pathological samples. The incorporation of either 14C-lysine or 14C-glucosamine into proteins of the vessel wall was also studied. In addition, high amounts of elastase-type activity was extractable from pathological aorta specimens which may contribute significantly to the loss of elastic tissue evidenced by ultra-structural studies and confirmed by the biochemical technics. We propose therefore that increased elastase-type protease activity in these pathological aortas does significantly contribute to the weakening of the aortic wall and also may well be the main cause of the rupture of aneurysms observed occasionally.