The bronchodilator effect of three successive stable plasma terbutaline levels was studied in 10 children with asthma. Each terbutaline plateau was achieved by giving a rapid intravenous infusion of terbutaline, 0.9 microgram/kg, followed by a continuous infusion for 2 hours. Mean plasma terbutaline concentrations (18, 36, and 53 nmol/L at the three plateaus) were found to correlate linearly with the maintenance dose of terbutaline (2.4, 4.5, and 6.3 micrograms/kg/hr, respectively). Mean forced expiratory volume in 1 second increased from 65% to 96%, and mean forced mid-expiratory flow from 32% to 71% of the predicted normal value during the study (p less than 0.01); maximum bronchodilation was obtained at mean terbutaline levels of about 30 nmol/L (range 20 to 60). Effective plasma terbutaline levels were associated with side effects such as headache and tremor in all patients. In addition, heart rate increased from 84 to 116 beats/min, systolic blood pressure rose from 115 to 129 mm Hg, and diastolic blood pressure dropped from 72 to 61 mm Hg during the study. We conclude that a loading dose of 2 micrograms terbutaline per kilogram of body weight over 5 minutes, followed by a continuous infusion of 4.5 micrograms terbutaline per kilogram per hour, is suitable for treatment of severe bronchoconstriction in children. Because of interindividual variations in drug metabolism and clinical effect, dose adjustment should be evaluated at regular intervals.