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Terbutaline, a new bronchodilator drug reported to have selective affinity for beta 2-adrenergic receptors, was compared with epinephrine in the treatment of 49 adult subjects with acute bronchial asthma. Under double-blind conditions, 24 subjects received 1.0 mg of terbutaline sulfate, and 25 subjects received 0.5 mg of epinephrine hydrochloride subcutaneously. Spirometric measurements, heart rate, and blood pressure, as well as subjective responses, were recorded prior to, and then at 5, 15, 30, 60, and 120 minutes after administration of the drug. The results indicate that terbutaline is an effective bronchodilator drug in subjects with acute asthma; however, the heart rate rose significantly after administration of terbutaline, with a maximal increase of 25 percent above control. Review of the literature reveals that tachycardia is a consistent finding when subcutaneous doses of terbutaline in excess of 0.25 mg are administered. Stimulation of beta 1-adrenergic receptors in the heart appears to be the most important factor involved in this response. A lesser cardioaccelerator effect was observed after administering epinephrine in a dose producing an equivalent degree of bronchodilatation.