Studies of the metabolic fate of bronchodilator drugs such as terbutaline and isoprenaline lead to the conclusion that less than 10% of an inhaled dose is absorbed from the airways. The remaining 90% is swallowed but is largely prevented from reaching the systemic circulation because of metabolism in the gut wall and liver. However, it is the minor fraction reaching the airways which produces the therapeutic effect. Activated charcoal has been used to prevent absorption of terbutaline from the Gl tract thereby allowing more accurate estimates to be made of pulmonary absorption. The results confirm previous estimates but, in addition, indicate considerable inter-subject variation in pulmonary absorption.