Quinapril pharmacokinetics were studied in infants and children using a novel study design that allowed substitution of quinapril for one dose of the current chronic angiotensin-converting enzyme (ACE) inhibitor treatment. A total of 24 patients ranging in age from 2.5 to 82 months who were receiving an ACE inhibitor held their usual treatment on the study day and received a 0.2-mg/kg dose of quinapril syrup. Blood samples were collected through 24 hours postdose, and plasma was analyzed for quinapril and its active metabolite, quinaprilat. Quinapril was rapidly converted to quinaprilat. Quinaprilat concentrations generally peaked 1 to 2 hours postdose and declined with a mean half-life of 2.30 hours. Dosing on a mg/kg basis resulted in quinaprilat AUC and Cmax values that were generally comparable across the age range of patients in this study. The overall mean AUC0-infinity was 993 ng.h/mL (range: 533-1523), and mean Cmax was 260 ng/mL (range: 70.0-445.5). Quinaprilat CL/F correlated well with body size (body surface area or weight) and creatinine clearance (mL/min). Pharmacokinetic results after a 0.2-mg/kg dose in infants and children are comparable to those observed following a 10-mg dose in adults.