The tools which have led to a better understanding of the role of the renin-angiotensin system in determining the tone of peripheral arteries in cardiovascular disease consist of highly specific and sensitive methods to measure the different components of the renin-angiotensin system and in specific probes used to inhibit the renin-angiotensin system at several points of the enzymatic cascade. The therapeutic efficacy of these more and more specific probes undoubtedly provides strong circumstantial evidence that angiotensin II plays an important role in determining peripheral vascular tone. While this has considerably increased our understanding of the regulatory mechanism involved in blood pressure maintenance in normotensive subjects and hypertensive patients, the role of the renin-angiotensin system as an aetiological factor in the development of essential hypertension still remains unclear.