Saline and 2.5 mg of isoproterenol were administered by nebulization to 18 subjects with chronic bronchitis. It was found that airway resistance lessened with isoproterenol hydrochloride therapy, but not with saline. Arterial oxygen and carbon dioxide tension fell insignificantly immediately following nebulization of both substances. Thirty minutes following nebulization blood gas levels had returned to baseline. Pulse rate and minute ventilation were not affected by nebulization of either saline or isoproterenol. We conclude that the immediate postnebulization effects on blood gas levels are due to nebulization and not to isoproterenol therapy. The action of nebulized isoproterenol appears to be limited to the bronchial tree.