Procaterol, a new beta-adrenergic agonist, was tested in two different doses in patients with reversible airway obstruction. Single oral doses of 0.05 or 0.1 mg produce an increase in measured expiratory flow rates, and the peak and duration of action are both dose related. Side effects reported during this one-day study were similar to those described with other compounds of this class; they were considered to be mild by the patients and the investigator. From these results, further investigation of this agent in bronchoconstrictive disease appears warranted.