The effects of aerosolized metaproterenol (1.30 mg) and isoetharine (1.02 mg) on nonspecific airway responsiveness to inhaled methacholine were quantitated and compared in 6 asthmatic subjects for 4 h after dosing. Both metaproterenol and isoetharine demonstrated bronchodilatation and reduced airway responsiveness during the first hour. Isoetharine had lost its bronchodilator effects and its effects on airway responsiveness by 2.25 h. The effects of metaproterenol on airway responsiveness had decreased by half at 2.25 h, while its bronchodilator effect remained maximal. Metaproterenol had no effect on airway responsiveness by 4 h, despite persisting bronchodilatation. Thus, for metaproterenol, there is disparity between duration of bronchodilation and suppression of airway responsiveness. Because methacholine sensitivity is a measure of nonspecific airway responsiveness, which appears to correlate with the severity of asthma, this methodology may provide more relevant information for assessment of duration of effect and estimation of appropriate dosing intervals for maintenance therapy than do more traditional methods.