Apafant (4-(2-chlorophenyl)-9-methyl-2-(3-morpholino-3-oxopropyl)-6H-thieno[3,2- f] [1,2,4]triazolo[4,3-a][1,4] diazepine, CAS 105219-56-5, WEB 2086), as a specific platelet activating factor (PAF) antagonist, inhibited PAF-induced increases of bronchial inflation pressure (delta Pi), pulmonary artery perfusion pressure (delta Pp) and microvascular permeability (wet-to-dry lung weight ratios), dose-dependently, in rats. Apafant also inhibited antigen-induced increase of delta pi, delta Pp and microvascular permeability in passively sensitized rats. Ozagrel also inhibited PAF- and antigen-induced increase of delta Pi, delta Pp and microvascular permeability. Apafant almost completely inhibited the increase of intratracheal pressure and microvascular permeability, but incompletely inhibited the increase of pulmonary artery pressure. At 1 microgram/ml, the effects of ozagrel were almost comparable to that of apafant at the same concentration, but the inhibitory effect on intratracheal pressure was less than that of apafant. Apafant inhibited PAF-induced increase in perfusate of thromboxane (TX) B2 and leukotrine C4/D4E4 (LTs), and antigen-induced increase of TXB2, LTs, PAF and histamine. Ozagrel also inhibited the PAF-induced increase of TXB2, but not the increase of LTs. Apafant inhibited antigen-induced increase of TXB2 and LTs more strongly than PAF-induced increase. The order of inhibitory effects of apafant against generation and release of chemical mediators was TXB2, LTs, PAF and histamine. These findings suggest that TXA2, LTs and PAF may contribute to the increase of intratracheal pressure and microvascular permeability, and histamine may contribute to the increase of vascular resistance in rats. Apafant may inhibit bronchopulmonary responses through PAF receptor antagonism. In addition, apafant can be considered to be useful for the treatment of some allergic diseases when the drug is employed in clinical use.