Chronic blockade of the renin angiotensin system became possible when orally active inhibitors of angiotensin converting enzyme, the enzyme which catalyzes the transformation of angiotensin I into angiotensin II, were synthetized. Two compounds, captopril and enalapril, have been investigated in clinical studies. The decrease of the pressor response to exogenous angiotensin I and of the circulating levels of angiotensin II following administration of these inhibitors has been demonstrated to be directly related to the degree of suppression of plasma angiotensin converting enzyme activity. These inhibitors have been shown to normalize blood pressure alone in some hypertensive patients whereas in many others, satisfactory blood pressure control can be achieved only after the addition of a diuretic. Captopril and enalapril also markedly improve cardiac function of patients with chronic congestive heart failure. Chronic blockade of the renin angiotensin system has therefore provided an interesting new approach to the treatment of clinical hypertension and heart failure.