Advanced aortic and coronary atherosclerosis was produced in rhesus monkeys by means of two procedures' (a) high fat and cholesterol feeding for 7 months, and (b) this diet coupled with daily i.v. injection of adrenaline (50 micrograms/kg body weight). A total of 83% of the monkeys subjected to procedure (b) developed markedly advanced atherosclerosis in the form of fibrous plaques in the aorta and coronary artery, while these lesions were much less frequent in the other group. The ratio of total to free serum cholesterol, significantly increased and the aortic cholesterol content was very high in monkeys subjected to both the atherogenic diet and adrenaline injections. It is suggested that catecholamines cause vascular injury and, in the presence of hyperlipidaemia, cause accelerated and aggravated atherosclerosis.