We studied the relationships between adrenaline and noradrenaline and factors associated with arteriosclerosis to determine whether catecholamines contribute to the atherogenetic process. We investigated the effects of adrenaline and noradrenaline on cultures of vessel wall cells from rats and analyzed plasma catecholamine levels in humans exposed to atherogenic risk factors, undergoing hemodialysis treatment or following myocardial infarction or stroke. I. Cultured endothelial and smooth muscle cells from vessel walls exhibited enhanced proliferation when exposed to adrenaline or noradrenaline. This indicates that catecholamines trigger the activation of vascular wall cells in vitro. Such activation, the unspecific mesenchymal reaction, is the predominant characteristic change in early atherogenesis. II. In individuals subjected to the atherogenic risk factors smoking, essential hypertension and mental stress, plasma adrenaline concentrations were statistically significantly elevated. Mental stress also caused significantly elevated plasma noradrenaline levels. Plasma noradrenaline concentrations were also elevated in smoking and hypertensive individuals when compared with certain controls, but the differences failed to be statistically significant. III. In dialysis patients, plasma adrenaline and noradrenaline concentrations showed a positive correlation with the activity of the sclerotic process; i.e., plasma catecholamine concentrations increased with the severity of the disease. IV. Patients with persisting arteriosclerotic vascular disease, i.e., patients who had had a myocardial infarction or stroke, had significantly elevated plasma adrenaline and/or noradrenaline levels as late as one year after the event. The results of our investigations suggest that adrenaline and noradrenaline may act as chemical mediators during atherogenesis in man, thus contributing to the development and subsequent complications of arteriosclerosis.