Pleurisy was induced in rats after intrapleural injection of 2% λ-carrageenin. The volume of the pleural exudate increased rapidly between 1 and 3 h, continuing to increase until 7 h. The dye amounts exuded into the pleural cavity for 20 min after 5% pontamine sky blue (60 mg/kg, i.v.) increased markedly at 1 to 3 h, then they kept constant until 6 h, decreasing thereafter. The PGE level in the pleural fluid increased rapidly during the period from 1 to 3 h, when the exudate commenced to accumulate, and then decreased slightly until 7 h. The main PG in the pleural fluid was recognized as PGE2 on thin-layer chromatography. The pretreatment of rats with a PG synthetase inhibitor, indomethacin (5 mg/kg, i.p.), 30 min before carrageenin administration resulted in the significant reduction of dye leakage at 1 h and of the pleural fluid exudation at 1 to 3 h. The intravenous injection into rats of stem bromelain (10 mg/kg, i.v.), a SH-protease from pineapples, caused the depletion of high molecular weight kininogen in plasma without effect on the content of low molecular weight kininogen. The pretreatment of rats with bromelain 30 min before carrageenin caused a marked reduction in the dye exudation during the periods from 1 to 3 h and 5 to 6 h. The accumulation of the exudate was strongly suppressed at the same periods. Although each treatment by itself did not completely abolish the dye and fluid exudation, the treatment of rats simultaneously with indomethacin and bromelain resulted in suppression of dye leakage from 1 to 6 h and practically no fluid accumulation was observed until 4 h. These results strongly indicate that prostaglandin is released in combination with bradykinin at the period when the pleural exudate commences to accumulate.