The renin-angiotensin system is the most important hormonal system in the control of blood pressure. It can be blocked at several steps in its reaction pathway. Angiotensin-converting enzyme, or kininase II, is responsible for catalysing the formation of vasoconstrictor angiotensin II as well as for the breakdown of the vasodilator bradykinin. With the advent of specific orally active angiotensin-converting enzyme inhibitors (captopril and enalapril) a practical and effective means of blocking angiotensin formation is now clinically available. ACE inhibitors are effective long-term therapy in patients with essential hypertension. They are particularly useful in the treatment of severe accelerated drug-resistant hypertension and renovascular hypertension, and the agents of choice for afterload reduction in severe congestive cardiac failure. They have several pharmacological advantages, important in anaesthetic practice, over other hypotensive agents. They have no central nervous or autonomic nervous system effects. Thus, patients being treated with ACE inhibitors retain normal sympathetic cardiovascular responses.