Inhibitors of angiotensin converting enzyme block the conversion of angiotensin I to angiotensin II, the active hormone of the renin-angiotensin system. This inhibition leads to a reduction in angiotensin-mediated vasoconstriction and aldosterone production. Although converting enzyme inhibitors have other potential metabolic effects, their beneficial effects in hypertension and congestive heart failure appear to be, in large part, related to their ability to reduce angiotensin II. This causes an increase in plasma renin levels and a fall in plasma and urine aldosterone, which can be sustained for many years. As a consequence, converting enzyme inhibitors produce mild natriuresis and positive potassium balance. At conventionally used doses, enalapril more completely prevents posture-induced increases in aldosterone than does captopril, probably reflecting more complete inhibition of angiotensin II formation in vivo.