This study was designed to examine whether an inhaled β2-agonist, procaterol, inhibits thromboxane A2 (TXA2) production induced by antigen challenge in passively sensitized guinea-pigs in vivo. Antigen-induced bronchoconstriction was markedly inhibited by pre-treatment with procaterol. Inhaled procaterol significantly reduced in a dose-dependent manner the increment in TXB2 concentration in bronchoalveolar lavage fluid obtained 5 min after antigen challenge. Aerosol administration of procaterol significantly inhibited bronchoconstriction induced by inhaled hislainine. These results suggest that inhalation of procalerol has an inhibitory effect on antigen-induced TXA2 production as well as a protective effect against bronchoconstriction induced by bronchoactive agents.