Ipratropium bromide (also known as Sch 1000) is a new atropine-like bronchodilator drug whose mechanism of action is via an anticholinergic pathway and may decrease cyclic guanosine monophosphate. Although of established efficacy in asthma, there are no studies of the use of ipratropium in patients with chronic bronchitis. The single metered aerosol doses of 10 mug, 20 mug, 40 mug and 80 mug of ipratropium bromide, 75 mug and 150 mug of isoproterenol, and placebo were studied in 20 adult patients, half with asthma and half with chronic bronchitis. To qualify, all patients demonstrated at least 20% improvement in the forced expiratory volume in one second while in the drug-free state when tested with isoproterenol. All subjects were tested for six hours with each agent in a double-blind crossover design. The dose-response aspects of the study indicate that in bronchial asthma the optimal range of dosage is 40 mug to 80 mug of ipratropium bromide. These doses are superior to isoproterenol in duration of action. In chronic bronchitis, all doses of ipratropium showed prolonged efficacy, but 80 mug was superior. Isoproterenol lacked this sustained efficacy. No significant alteration in pulse or blood pressure was observed. Ipratropium appears to be an important addition to the bronchodilator agents used in isoproterenol-responsive obstructive pulmonary disease.