A survey has been made in adult dogs of the collagen and elastin content of the walls of arteries varying in size from the thoracic aorta to the saphenous artery. In all arteries, except the smallest, collagen and elastin together formed about 50% of the dry weight. In the smallest the proportion was somewhat higher. On the basis of relative proportions of elastin and collagen the systemic arterial tree was found to be divided rather sharply into two regions: in the intrathoracic aorta there was about twice as much elastin as collagen in the wall; in all other vessels the relation was reversed, there being about twice as much collagen as elastin. The transition between the more elastic intrathoracic aorta and the more collagenous extrathoracic vessels was abrupt and took place over a distance of only 5 cm or so. The pulmonary artery resembled the extrathoracic vessels rather than the thoracic aorta in ratio of elastin to collagen. Newborn and young puppies showed a similar difference to adults between thoracic and abdominal aorta, although we did not investigate the relation between the two vessels in detail.