In a single-blind study the short-term effects of oral administration of Th1165a (5, 10, 15, and 20 mg), metaproterenol sulfate (Alupent) (20 mg), and placebo on ventilatory function, pulse rate, and systolic and diastolic blood pressure were compared over a period of six hours in ten patients with stable, reversible obstructive airway disease. Both Th1165a (5, 10, 15 and 20 mg) and metaproterenol administration caused significant bronchodilation of rapid onset (30 minutes), but the bronchodilator effect of Th1165a (10, 15, and 20 mg) was greater and lasted longer (six hours vs three hours) than that of metaproterenol. A dose-dependent bronchodilator effect was recognizable after administration of Th1165a. The 20-mg dose of metaproterenol sulfate and the 5-mg and 10-mg doses of Th1165a produced minimal side effects. Larger doses (15 and 20 mg of Th1165a caused significant increases in pulse rate. Mild and transient tremors were the most common side effect after administration of Th1165a.