Recently there has been extensive development of orally active angiotensin converting enzyme (ACE) inhibitors in addition to those already marketed, for example, captopril, enalapril, lisinopril and ramipril. It was initially thought that ACE inhibitors were likely to be most useful as antihypertensive agents in conditions in which circulating renin and angiotensin II were elevated. However, it is now clear that they can also lower arterial pressure when plasma renin is not high. In addition, they have beneficial effects in cardiac failure.Thus, captopril, enalapril, lisinopril and ramipril can be used in the treatment of mild to moderate hypertension either alone or in conjunction with diuretics or calcium antagonists. Broadly speaking, efficacy appears to be similar to that of &bgr;-blockers or diuretics. Unfortunately, however, there are no long term studies comparing one ACE inhibitor with another or with other classes of antihypertensive agents. Furthermore, there are no prognostic studies which show that use of ACE inhibitors reduces morbidity or mortality in hypertension.Many new ACE inhibitors are undergoing clinical assessment, including alacepril, cilazapril, fosenopril, perindopril, quinapril and ramipril. The drugs vary, in that some exist in the active form whereas others are prodrugs which are converted to the active agent following absorption. In addition they each possess one of several ligands, for example, carboxyl, phosphinyl or sulfhydryl groups, and so vary in their affinity for ACE. Although many of these agents are renally excreted, a small number are metabolised via the liver (e.g. quinapril and spirapril) and this may prove advantageous in the presence of renal impairment.In common with captopril and enalapril, the new ACE inhibitors inhibit the reninangiotensin system and initial results suggest that they are effective in lowering blood pressure in essential hypertension. Furthermore, they reduce systemic vascular resistance in the absence of a reflex tachycardia.There are a number of adverse effects which are attributable to the pharmacological mechanism of the ACE inhibitors as a group; these include hypotension, particularly in patients with high renin levels, prior diuretic use, renal impairment or in the elderly. Additional adverse effects may relate to chemical structure. The high incidence of adverse effects noted in early studies related to excess dosage and to the presence of a sulfhydryl group, which the more recently developed ACE inhibitors lack. The adverse effects most commonly reported with established and new ACE inhibitors include headache and fatigue, cough, skin rashes, hypotension and diarrhoea. As a group, ACE inhibitors have an acceptable but not negligible adverse effect burden. They need to be introduced cautiously if plasma renin is likely to be raised, and in the elderly, both circumstances in which first dose hypotension may occur. They can depress renal function in cases of bilateral renal artery stenosis and in cardiac failure.The ACE inhibitors have now been shown to be effective in the treatment of moderate hypertension, in supporting the ailing heart, and in combination with various other classes of antihypertensive drugs. While caution is needed with any new type of agent until wide experience has accumulated, ACE inhibitors clearly possess several features which make them attractive candidates for the initiation of antihypertensive therapy.