Essential hypertension is a quantitative abnormality, the pathological effects and risks increasing with the blood pressure level. In Western countries blood pressure rises with age in most individuals, so essential hypertension is more frequent in middle and older age groups. It is likely that an individual's blood pressure level is determined by many interacting factors. These include heredity, which probably acts multifactorially, and many environment influences, including psychological stress and obesity. Specific factors may be of varying importance in different individuals and in different populations. Several physiological mechanisms control the blood pressure level and may be altered in essential hypertension. In early hypertension sympathetic nervous activity is sometimes increased, although in long-standing hypertension this is less marked. Cardiac output may be increased in borderline hypertension but is normal in established hypertension, when total peripheral resistance is increased. Total exchangeable sodium is normal, while the renal pressure-natriuresis balance is altered, so that for a given pressure the hypertension kidney excretes less sodium. In some patients, plasma renin is low, probably as a result of renal adaption to prolonged hypertension. The pathogenic sequence in essential hypertension is uncertain. Increased autonomic activity may cause vasoconstriction in renal and other arterioles and increase cardiac output, leading to a rise in blood pressure. Elevated pressure itself produces structural changes in the resistance vessels, including those of the kidney, which eventually maintain the hypertension even when the initiating stimulus is removed. The way in which heredity and environment influence pathogenic mechanism is also uncertain. Heredity might, for example, influence the autonomic response to stress or the liability to irreversible changes in the resistance vessels or in the kidney. Environmental factors may also increase autonomic activity, enhance vascular reactivity or alter renal function.