The control of arterial pressure in normal and pathological states is governed by many mechanisms. The renin-angiotensin-aldosterone system is one mechanism which may be activated. The formation of the potent vasoconstrictor angiotensin II and the synthesis of aldosterone provide a means of arterial pressure regulation. With the recent introduction of specific agents which block the renin-angiotensin system, new appreciation of the role of this system is being formed. It is premature to predict whether we, as anaesthetists, will use such agents in our anaesthetic practice. However, we will see some patients who present for surgery taking such medication. How this will influence the patient's response to surgery is unknown. Inhibition of the renin-angiotensin system may allow better perfusion of vital organs. It may, however, be detrimental to the patient who needs this defense mechanism for support of his arterial pressure. At the present time these are all unsettled questions. Through the use of specific inhibitors of the renin-angiotensin system its importance will be defined, much as alpha- and beta-adrenoceptor antagonists have clarified our understanding of the sympathetic nervous system.