The distribution of nebulized isoproterenol and its effect on regional ventilation and perfusion were studied. Seven patients with intrinsic asthma underwent spirometry, arterial blood gas determinations, and xenon-133 ventilation and perfusion lung scans both before and after an intermittent positive pressure breathing (IPPB) treatment with 2.5 mg of isoproterenol delivered through an ultrasonic nebulizer. The isoproterenol was accompanied by a tracer dose of technetium-99m as pertechnetate. Forced expiratory volume in one second (FEV1) and vital capacity (VC) improved slightly. Changes in arterial oxygen tension (Pao2) were variable and correlated with changes in total alveolar ventilation. Changes in alveolar-arterial oxygen tension difference ([A-a] Po2) were not correlated with detectable changes in over-all matching of ventilation and perfusion ( ). Isoproterenol was unevenly distributed without regard to the pretreatment distribution of ventilation. Nebulized isoproterenol produced no detectable regional effect on ventilation, perfusion, or ratio.