The dose response characteristics and side effects of three doses of oral metaproterenol sulfate (MS) (0.4, 0.6, and 0.8 mg/kg) were compared with a placebo in ten asthmatic children (3 to 6 years). The bronchodilator efficacy was objectively assessed by evaluating the decrease in mean total respiratory resistance (MTRR) using Oscillaire. The mean percentage changes from baseline in MTRR showed that MS in all three doses was more effective than placebo. Lack of objective demonstration of bronchodilator efficacy, concern for possible side effects and uncertainty of the precise dosage for adequate bronchodilation are some of the factors that account for the lack of specific dosage recommendations for bronchodilators such as oral metaproterenol sulfate in children under 6 years of age. Until recently, it was not possible to objectively measure the response to bronchodilators in children below 6 years of age because of their inability to reliably perform forced expiratory maneuvers required for spirometry and their unwillingness to have airway resistance (or conductance) measurements by body plethysmography. Direct measurements of airflow resistance have been made in children with a pneumotachograph at the mouth and a balloon in the esophagus to record transpleural pressure changes; however, these techniques are invasive and require sedation. Total respiratory resistance and reactance of the respiratory system, as measured by forced oscillation technique, are pulmonary function tests that can be performed in young children who are unable to cooperate with conventional spirometric maneuvers.