The sympathetic nervous system is of major importance for the regulation of several physiologic functions. Drugs that inhibit the actions of catecholamines and adrenergic drugs are used in the treatment of many clinical disorders. The potential role of catecholamines in a number of human diseases has, however, until recent years been studied to a limited extent only due to lack of methods for quantitation of sympathetic nervous activity. After the development of enzymatic isotope-derivative assays, reliable measurements of noradrenaline and adrenaline became available. Studies in man have shown that plasma noradrenaline is an index of sympathetic nervous activity. The present survey deals with noradrenaline and adrenaline concentrations in blood, tissue, and cerebrospinal fluid in a number of clinical disorders viz. arterial hypertension, duodenal ulcer, malignant tumors, primary depressive illness, and ketotic hypoglycemia.