This review summarizes the available medical literature about plasma norepinephrine, which as been used as an indicator of sympathetic neural activity in clinical cardiology. Plasma norepinephrine levels are elevated myocardial infarction and congestive heart failure, and the norepinephrine concentration varies with severity of disease. Patients with ischemic heart disease at rest show essentially normal plasma norepinephrine, but no studies have assessed norepinephrine levels during spontaneously occurring typical angina pectoris. Plasma norepinephrine also is increased during hypertension occurring after coronary bypass surgery or repair of aortic coarctation. Propranolol increases plasma norepinephrine, and acute withdrawal of propranolol does not. Sodium restriction increases plasma norepinephrine in healthy persons, but no information is available about its effect on patients with congestive heart failure. Insufficient data are available to make strong inferences about sympathetic activity in cardiomyopathy, essential hypertension or pulmonary hypertension, and little or no information is available about plasma norepinephrine in ventricular fibrillation without myocardial infarction, the mitral valve prolapse syndrome, digoxin effect, syndromes associated with prolonged electrocardiographic Q-T interval and the hyperkinetic heart syndrome.