The pharmacokinetics of isoproterenol (ISO) in infants and children have never been reported. The authors studied ISO pharmacokinetics in two disparate groups of pediatric intensive care unit patients: postoperative cardiac patients (POC, n = 10), and reactive airway disease patients (RAD, n = 9). In all, 44 blood samples were taken at steady‐state from the 19 patients, whereas from 15 patients samples were also taken just before and after discontinuation of ISO infusion. There were 12 male and 7 female patients in the study, and their ages ranged from 2 days to 14 years. The average ISO dosing rate was 0.30 μg/kg/minute for the whole study population, ranging from 0.01 to 5.5 μg/kg/minute. The POC patients received a significantly lower dosing rate than the RAD patients (0.029 ± 0.002 vs 0.50 ± 0.21 μg/kg/minute, P<.0001); the average steady‐state plasma concentrations of ISO were also lower in the POC patients (1.3 ± 0.3 versus 13.9 ± 4.9 ng/mL, P<.0001). The steady‐state plasma concentration, normalized to a dosing rate of .05 ng/kg/minute, was 1.9 ± 0.3 ng/mL for all patients, and the clearance was 42.5 ± 5.0 mg/kg/minute. Postoperative cardiac patients had a significant higher normalized steady‐state plasma concentration and moderately significant lower clearance than did RAD patients (2.1 ± 0.3 versus 1.7 ± 0.4 ng/mL, P<.05 and 33.2 ± 4.9 versus 48.4 ± 7.3, P<.06, respectively). The average plasma half‐life of ISO was 4.2 ± 1.5 minutes, and the volume of distribution was 216 ± 57 mg/kg. This study suggests that the pharmacokinetics of ISO in children is highly variable and may vary with serum concentration. The apparent higher clearance of ISO in the RAD patients may be related to an improved perfusion status relative to the POC patients or to a possible influence of no