1 The novel thromboxane (TX) antagonist, BAY u3405, has been evaluated against bronchoconstriction induced by the TXA2 mimetic U-46619, prostaglandin D2 (PGD2), 5-hydroxytryptamine (5-HT), leukotriene D4 (LTD4) and histamine in the guinea-pig in vivo by use 6 The action of BAY u3405 (l0mgkg-,p.o.) was long lasting, causing significant inhibition of U-46619-induced bronchoconstriction 7 h after dosing. 7 At 1 mg kg-1, i.v., a dose that abolished the response to U-46619 and PGD2, BAY u3405 had no effect on histamine-, 5-HT-or LTD4-induced bronchoconstriction. 8 BAY u3405 potently and selectively antagonized U-46619-or PGD2-induced bronchoconstriction in the Konzett-Rbssler model of guinea-pig lung function. It should therefore prove to be a useful tool for defining the role of TXA2-and PGD2 in airway diseases such as asthma.