The catecholamines adrenaline , and the thyroid hormone play an important role in regulating circulation owing to their influence upon the vessel wall. Apart from this physiological effect, the hormones, if given in high doses, are able to induce severe pathological changes in the arterial wall. In recent years there has been an increasing interest in the vascular connective -tissue changes in arteriosclerosis. It has revived Virchow's theory that arteriosclerosis is due primarily to non-specifc damage to the arterial wall, which may elicit inflammatory changes secondarily predisposing to depositions of lipids. From this point of view investigations on the reaction of the arterial wall to injury seems to be of utmost importance in the study of human arteriosclerosis. As a model for experiments of this nature, the arterial changes induced by catecholamines and thyroid hormone afford the advantage of being reproducible and rapidly developing. Further, the changes bear resemblance to human arteriosclerosis. After some pathogenetic considerations, the morphological and biochemical changes in rabbit aorta following administration of large doses of catecholamines and thyroid hormone are described. The changes affect primarily the media, but intimal thickening is not uncommon. Characteristic are focal necroses, calcifications, accumulations of amorphous, metachromaticallystaining, ground substance, and, in the older lesions, formation of collagen fibers. The lesions are not primarily accompanied by lipid deposits, but predispose to such depositions. Biochemical analysis reveals an increase in the amount of muco-polysaccharides in the arterial wall, the total amount as well as the acid fraction, an increase in the mucopolysaccharide to collagen ratio and in the synthesis of the sulphated acid mucopolysaccharides. The older lesions show an increase in the collagen content and a decreasing synthesis of sulphomucopolysaccharides. There is only a limited parallelism between the morphological and biochemical changes, primarily due to the dependence of the biochemical changes upon the time of injury. Comparison of the morphological and biochemical aortic changes induced by catecholamines and thyroid hormone with the changes in the arterial wall induced by a number of other entirely different agents confirms the hypothesis, that the reaction of the arterial wall to damage is histologically as well as biochemically non-specific and uniform, largely independent of the nature of the damaging agent. The morphological and biochemical changes in the arterial wall elicited by injury closely parallels the changes observed in cutaneous wounds and in all granulation tissue. Accordingly, the arterial changes may be interpreted as processes of repair. A comparison of the morphological and biochemical changes in human arteriosclerosis and in experimentally induced arterial lesions shows, that an essential feature of human arteriosclerosis is probably healing processes, evoked by various forms of damage to the arterial wall.