The renin-angiotensin system is known to respond to various stimuli with a marked increase in its activity. This phenomenon is well known in relationship to changes in sodium balance. Thus, the antihypertensive effect of diuretics is often blunted or even abolished by an excessive renin response. Water deprivation and alpha-adrenoceptor blockade also elicit considerable renin responses, which as a result become a predominant factor in blood pressure maintenance. Accordingly, the renin system plays a very important role in blood pressure regulation not just because it is spontaneously activated under steady-state conditions, but even more because of its responsiveness to pharmacological and other stimuli. This may explain the surprising efficacy of angiotensin converting enzyme inhibitors, which block the generation of angiotensin II, and thereby all vascular effects of spontaneously elevated renin levels as well as of any renin response.