The pharmacokinetics of angiotension converting enzyme (ACE) inhibitors enalapril (10 mg orally) and its active metabolite, enalaprilat (10 mg intravenously) were studied in nine young healthy volunteers aged 22-30 years and nine sex matched elderly subjects aged 65-73 years. After both drugs, a biexponential curve was fitted to the decline in plasma enalaprilat concentration. Area under the plasma concentration-time curve (AUC) was greater in the elderly for both drugs. Clearance (CL) and clearance/bioavailability (CL/F) were less in the elderly for enalaprilat and enalapril, respectively. There was no difference in F between young (0.62 +/- 0.16) and elderly subjects (0.61 +/- 0.15). Enalaprilat CL and enalapril CL/F were significantly and positively correlated to endogenous creatinine clearance. There was a significant difference in the weight corrected volume of distribution at steady state after enalaprilat between the young and elderly (P less than 0.02). The relationship between plasma enalaprilat concentrations and percentage ACE inhibition, using the Hill equation, showed no difference in the sensitivity to ACE inhibition between the young and the elderly group. The pharmacokinetic differences observed are likely to be related to an age dependent decline in renal function as well as changes in body composition. Kinetic differences partly explain the greater pharmacodynamic response in the elderly.