Studies were performed in vitro on cylindrical segments of 56 canine common carotid arteries, 32 human external iliac arteries, nine internal iliac arteries, and ten common iliac arteries, using purified elastase and purified collagenase. Treatment with elastase caused the canine vessels to dilate but to remain intact. Similar results were obtained with the human vessels, except that treatment with elastase caused only slight dilation. All canine and human vessels treated with collagenase ruptured. We concluded that wall integrity depends on intact collagen rather than elastin. Comparison between external iliac arteries and internal and common iliac arteries showed that the latter vessels exhibited dramatically greater dilatation and compliance changes after treatment with collagenase. This finding corresponds to the greater tendency of aneurysms to develop in internal and common iliac arteries.