In rat carrageenin pleurisy, dexamethasone (0.25 mg/kg p.o.) inhibited exudate accumulation in the pleural cavity throughout the period of testing while the non-steroidal anti-inflammatory drugs indomethacin (10 mg/kg p.o.) and BW755C (100 mg/kg p.o.) inhibited it for 6 and 9 h respectively after the intrapleural injection of carrageenin but not thereafter. These observations were confirmed by the finding that, of the treatments 6 h after carrageenin, only that with dexamethasone reduced exudate volume at 15 h after carrageenin. The anti-exudative action of dexamethasone at 15 h after carrageenin may be at least in part mediated by endogenous substance(s) formed through gene expression, because it was prevented by actinomycin D. Dexamethasone and BW755C markedly inhibited cell accumulation in the pleural exudate throughout the period tested, while indomethacin inhibited it significantly between 9-24 h after carrageenin.