Fifteen subjects with chronic bronchitis were treated with inhaled atropine sulfate, isoproterenol hydrochloride, or placebo in a double blind crossover trial. Each drug was inhaled 4 times per day for 3 weeks. Baseline forced expiratory flows, specific airway conductance, airway resistnace, and functional residual capacity were measured weekly. These measurements were repeated 15 min and 60 min after laboratory inhalation of each agent, using a separate double blind protocol. No significant differences were observed among the mean baseline function studies during the 3 treatment periods. There were no significant differences in sputum production, sputum thickness, dyspnea, or mouth dryness among the 3 treatment periods. Side effects were mild; isoproterenol treatment was associated with an increased frequency of palpitations, and atropine treatment was associated with difficulty in voiding. Significant increases in lung function were apparent in the measurements made 15 min and 60 min after inhalation of atropine or isoproterenol. Mean values of flow and specific airway conductance increased, and mean values of airway resistance and functional residual capacity decreased after inhalation of either drug. There was no significant difference between the effects of atropine and isoproterenol on lung function, although both differed significantly from the effects of placebo (P less than 0.001). Inhaled atropine sulfate was as effective a bronchodilator as inhaled isoproterenol hydrochloride, and was well tolerated during the 3-week trial of daily therapy.