Enalapril and enalaprilat concentrations were measured after enalapril maleate (0.05 to 0.3 mg/kg) was administered orally to 12 pediatric patients (age range, 10 days to 6 1/2 years) with congestive heart failure caused by congenital heart disease and compared with those obtained from seven normal adults (age range, 21 to 39 years). When normalized to the oral 1 mg/m2 dose of enalapril maleate, the mean +/- SD area under the serum concentration-time curve (AUC) of enalaprilat, a pharmacologically active angiotensin-converting enzyme inhibitor, did not differ significantly between the pediatric group aged > 20 days and adult group (83.1 +/- 47.0 versus 64.6 +/- 17.8 ng.hr/ml per 1 mg/m2). When normalized to the oral 0.1 mg/kg dose, the mean AUC was significantly (p < 0.05) smaller in this pediatric group than in the adult group (138.4 +/- 69.2 versus 245.7 +/- 61.8 ng.hr/ml per 0.1 mg/kg). The AUC observed in three younger (age < 20 days) subjects tended to be much greater compared with infants aged > 20 days. The mean AUC ratio of enalaprilat to enalapril was significantly (p < 0.05) lower in the older pediatric subgroup (2.0 +/- 1.0) than in the adult group (3.4 +/- 1.6), whereas the mean ratios were comparable between the two subdivided pediatric groups. The results suggest that the oral enalapril dose would be better determined on a body surface area rather than on a body weight basis in pediatric patients with congestive heart failure aged > 20 days. The oral dosage should be much reduced in infants with congestive heart failure aged < 20 days compared with those aged > 20 days.(ABSTRACT TRUNCATED AT 250 WORDS)