The angiotensin converting enzyme inhibitors are an important therapeutic advance in the treatment of patients with hypertension and congestive heart failure. In addition, they are useful pharmacological probes to assess the contribution of the renin-angiotensin system to circulatory homeostasis.Captopril was the first angiotensin converting enzyme inhibitor approved for use in patients with hypertension and congestive heart failure. It is rapidly absorbed from the gastrointestinal tract, with detectable plasma concentrations apparent as early as 15 minutes. The extent of absorption is between 60 and 75% of an oral dose and peak plasma concentrations occur after approximately one hour. Captopril is primarily excreted by the kidneys via renal tubular secretion. Renal excretion is rapid, with 90% completed in the first 4 hours. The elimination half-life for unchanged captopril is about 1.7 hours and is markedly increased in the presence of renal insufficiency. Once absorbed, captopril is extensively metabolised to several forms, including a disulphide dimer of captopril, a captopril-cysteine disulphide, and other mixed disulphides with endogenous thiol compounds. It is probable that captopril and its pool of metabolites undergo reversible interconversions.Pharmacokinetic properties of captopril in patients with uncomplicated hypertension appear to be the same as in healthy subjects. However, long term administration of captopril leads to increased concentrations of total captopril, probably from the accumulation of captopril metabolites. Despite the number of potential influences on pharmacokinetic properties in patients with congestive heart failure, due to the many abnormalities in gastrointestinal tract oedema and reductions in splanchnic and renal blood flow, the available data suggest that its pharmacokinetic properties in patients with congestive heart failure resemble those in healthy subjects. However, additional data are necessary to confirm this.Enalapril is the second angiotensin converting enzyme inhibitor to become available. Enalapril is a prodrug that is well absorbed from the gastrointestinal tract, with 60 to 70% of an oral dose being absorbed. However, enalapril must be converted by hepatic esterases to the active form, enalaprilat. After the oral administration of enalapril, the tmaxfor enalapril is one hour, but for enalaprilat it is 4 hours. There is a prolonged terminal elimination phase with enalaprilat being detectable as late as 96 hours after dosing. Thus, enalapril has a much longer duration of action than captopril. Like captopril, enalapril is primarily excreted by the kidneys. The de-esterification of enalapril to enalaprilat probably occurs in the liver by hepatic esterases. There does not appear to be any other metabolite of enalapril other than enalaprilat.Like captopril, the pharmacokinetic properties of enalapril in patients with uncomplicated hypertension appear similar to those in healthy subjects. The pharmacokinetics of enalapril in patients with congestive heart failure have not been adequately assessed. Enalaprilat, the active form of enalapril, is available for intravenous injection. It appears to be useful as a probe of the renin-angiotensin system and can rapidly lower blood pressure in hypertensive crises. It will also rapidly reduce systemic vascular resistance and increase cardiac output in patients with congestive heart failure.