The purpose of this study was to investigate whether procaterol (beta 2 agonist) inhibits the release of cyclooxygenase products in bronchoalveolar lavage fluid (BALF) during in vivo allergic bronchoconstriction in passively sensitized guinea pigs. Antigen-induced bronchoconstriction was significantly inhibited by pretreatment with procaterol. The concentrations of 6-keto-PGF1 alpha, PGF2 alpha and TXB2 in BALF significantly decreased in guinea pigs pretreated with inhaled procaterol. The concentration of TXB2, which is the stable metabolite of TXA2, a strong bronchoconstrictor, was especially markedly decreased compared with that in the control animals. These data indicate that procaterol (beta 2 agonist) may inhibit the release of chemical mediators during allergic reactions as well as directly decrease the contractility of airway smooth muscle. Consequently, regular inhalation of procaterol may be clinically useful for prophylaxis of bronchial asthma.