Eighteen critically ill postoperative patients < 1 yr of age were studied to determine the pharmacokinetics and adverse effects of amrinone. All patients had undergone cardiopulmonary bypass for repair of congenital heart lesions. Plasma samples were obtained every 12 h while patients were receiving amrinone to determine when steady state was achieved; samples were also obtained within 24 h after amrinone had been discontinued. Elimination half-life (T2), clearance, and volume of distribution were calculated from plasma amrinone concentrations, and the incidence of platelet transfusion was monitored. T2(22.2 vs. 6.8 h) and clearance (1.1 vs. 2.6 ml/min kg), but not the volume of distribution (1.8 vs. 1.6 L/kg), differed significantly in patients < 4 wk of age in comparison to patients >4 wk of age. A negative correlation between T2and age (r = -.79) was observed. Platelets were administered no more frequently in study patients than in a similar group that did not receive amrinone. To achieve the plasma concentration of amrinone that is therapeutic in adults, current dosage recommendations are inadequate in neonates and infants. Infants should receive an initial iv amrinone bolus of 3.0 to 4.5 mg/kg in divided doses followed by a continuous infusion of 10 μg/kg min, while neonates should receive a similar bolus followed by a continuous infusion of 3 to 5 μg/kg rnin.