To evaluate the pharmacokinetics of amrinone and its metabolites in neonates and infants after reconstructive surgery for congenital heart disease.Prospective study.Pediatric intensive care unit in a university hospital.Fifteen neonates aged less than 1 month with transposition of the great arteries and 14 infants aged 2 to 6 months with complete atrioventricular septal defect.Amrinone, loading dose of 2 mg/kg, was administered before weaning from cardiopulmonary bypass, followed by a maintenance infusion of 7.5 microg/kg/min.Blood samples to determine plasma concentrations of amrinone, N-acetylamrinone, and N-glycolylamrinone were drawn before amrinone administration, frequently after the loading dose, every 6 hours during the maintenance infusion, and until 48 hours after the end of the infusion. Amrinone clearance was 2.4 +/- 0.9 mL/kg/min in neonates and 3.2 +/- 1.2 mL/kg/min in infants (p < 0.05). The volume of distribution at steady-state was smaller (p < 0.05) in neonates than in infants. The elimination half-life of amrinone was 10.7 +/- 6.7 hours in neonates and 6.1 +/- 1.4 hours in infants (p < 0.05). There was a linear correlation between the clearance of amrinone and the body surface area (r = 0.67; p < 0.05). The ratio of the plasma concentration of N-acetylamrinone to that of amrinone did not differ between neonates and infants.Amrinone is eliminated at a slower rate in neonates than in infants. The rate of acetylation of amrinone appears to be similar; the differences in the elimination capacity of amrinone are mainly due to the immature renal function in neonates.