The introduction of orally active angiotensin converting enzyme (ACE) inhibitors has revolutionized the treatment of hypertensive disorders and provided an effective alternative for the management of congestive heart failure (CHF). By interfering with the formation of angiotensin II, the active agent of the renin system, ACE inhibitors block the system's vasoconstrictive and sodium-retaining effects, with a consequent reduction in systemic blood pressure. The net effect is improved blood flow and reduced cardiac work. Thus, ACE inhibitors are likely to improve cardiac work capacity and quality of life. Their mechanism of action differs markedly from that of traditional antihypertensive agents which lower blood pressure while reducing cardiac output and blood flow. Since the primary action of ACE inhibitors is to block the renin system, a dramatic response to monotherapy suggests a large renin factor while the lack of a response suggests a low-renin state more amenable to treatment with a diuretic or calcium antagonist. Because of their many attributes, ACE inhibitors are increasingly used as first-line therapy for the treatment of hypertension or CHF. The prototype orally active ACE inhibitor, captopril, is a sulfhydryl compound with a good safety profile at the recommended dosages but reported toxicity at higher dosages. Second-generation ACE inhibitors (eg, enalapril and quinapril) are more potent sulfhydryl-free esters with a greater affinity for the converting enzyme. These newer agents are pro-drugs requiring ester hydrolysis to form the active free acid compound.(ABSTRACT TRUNCATED AT 250 WORDS)