Zindotrine, a new bronchodilator, may be an alternative to theophylline in treating reversible airflow obstruction. Efficacy and cardiovascular effects of a single 300 mg oral dose of zindotrine were compared with placebo in a two-period, double-blind, crossover trial. Twelve subjects with airflow obstruction reversible after isoproterenol and theophylline completed the trial. Improvement in pulmonary function (forced vital capacity [FVC], forced expiratory volume in one second, and forced expiratory flow rate from 25 to 75 percent of FVC) was greater after zindotrine than with placebo. Pulmonary function tests increased 15 percent or more over baseline in 30 minutes after active drug, lasting up to 6 hours. Mild decreases in heart rate and mean blood pressure occurred after both treatments, with changes equal in both treatment groups. Six subjects had mild subjective side effects after zindotrine (headache, dizziness, vertigo, flushing, and heartburn) compared with one report of lightheadedness after placebo. A single dose of zindotrine 300 mg provides effective bronchodilator action with a relatively prolonged response and tolerable side effects.