Thromboxane A2(TxA2) has been implicated in the pathogenesis of airway hyperresponsiveness. The effects of inhaled TxA2 on human airway function have not been studied because of its short half-life. U46619 is a chemical that mimics the effects of TxA2. The purpose of this study was to evaluate the effects of inhaled U46619 on human airway function and methacholine airway responsiveness. Airway responsiveness to methacholine and U46619 was measured in 19 subjects (13 asthmatic and six normal) and expressed as the provocative concentration causing a 20% fall in FEV1 (PC20). On one day, methacholine alone was inhaled. On a second day, U46619 was inhaled, then 1 h later methacholine was inhaled. On a third day, U46619 was inhaled, then repeated 1 h later. In six subjects, the effects of isotonic saline or a subthreshold concentration of histamine or U46619 were examined on methacholine airway responsiveness. U46619 was 178 times more potent as a bronchoconstrictor than was methacholine. Airway responsiveness to methacholine was correlated to airway responsiveness to U46619 (r = 0.87, p = 0.001). Subthreshold concentrations of U46619, but not of histamine, increased methacholine airway responsiveness. The mean maximal fall in FEV1 after inhaled methacholine was 13.2% (SEM, 3.4%) after saline, 12.4% (SEM, 2.4%) after histamine, and 25.7% (SEM, 2.0%) after U46619 (p = 0.0004). This effect lasted less than 1 h. There was no tachyphylaxis to repeated inhalations of U46619. These results indicate that in human subjects inhaled U46619 is a potent bronchoconstrictor that, when present in the airways, can cause airway hyperresponsiveness to inhaled methacholine in asthmatic subjects.