To examine the additive properties and the sites of action of inhaled atropine sulfate (0.05 mg/kg of body weight) and terbutaline sulfate (0.005 mg/kg) in patients with chronic airflow obstruction, we tested these aerosols separately and together in a double-blind random sequence. Twelve patients with chronic bronchitis and perennial obstruction of airflow were studied by measuring three indices of efficacy (specific airway conductance [Gaw/V(L)], the forced expiratory volume in one second [FEV1] and the forced vital capacity [FVC]) and three indices of the site of action within the airway (Δ[Gaw/V(L)/FEV1], the difference between the change in forced expiratory flow at 75 percent of vital capacity and the change in forced expiratory flow at 25 percent of vital capacity, and the change in density dependence of maximal airflow at 50 percent of vital capacity). Both atropine and the combination of atropine and terbutaline improved all indices of efficacy significantly more than did terbutaline. With individual exceptions, the addition of terbutaline to atropine improved Gaw/V(L) but not forced airflow. All measures of site of action suggested an advantage for atropine in relatively proximal airways. These results indicate that combined therapy with β-adrenergic and anticholinergic bronchodilator drugs is marginally more effective than therapy with atropine alone in these patients and suggest that anticholinergic aerosols dilate larger airways more effectively than the β-agonists.