Forty-nine patients with increased airway resistance (AR) and ten normal men were studied before and after isoproterenol hydrochloride inhalation. They were subdivided on the basis of chest roentgenogram, sputum production, and history into asthma, chronic bronchitis, and emphysema groups. lsoproterenol hydrochloride increased neon dilution volume (VA) but not diffusing capacity (DL), which was much less in the emphysema, highest in the asthma, and intermediate in the chronic bronchitis group. Neon dilution volume/plethysmographic lung volume (TLC) and DL/VA were similarly affected, except that VA/TLC was not increased after isoproterenol administration in the emphysema group. These results, confirm the anatomic observation that the capillary bed is most abnormal in emphysema and suggest that AR, which determines pulmonary gas distribution, is less responsive to bronchodilators in emphysema. They also suggest that dilution volume added by bronchodilators may have less than usual blood-gas contact.