The injurious effect of platelet activating factor (PAF) on gastric mucosa was studied by measuring bleeding in the acid perfused stomach of anesthetized rats. The effect of PAF on gastric mucosal vascular permeability (GMVP) was assessed by dye-leakage in the saline perfused stomach of anesthetized rats. Intravenous infusion of PAF (100 ng/kg/min for 20 min) apparently caused gastric bleeding under the gastric luminal perfusion with 150 mM HCl solution,, with the peak response at 50-70 min; biopsy indicated the presence of mucosal lesions. GMVP was markedly increased with the peak response at 20-40 min. Pretreatment with CV-3988 (1 or 10 mg/kg, i.v.), a PAF antagonist, dose-dependently blocked the PAF-induced gastric bleeding and increase in GMVP. Pretreatment with hydrocortisone acetate (20 or 40 mg/kg, s.c.) reduced PAF-induced gastric bleeding and increase in GMVP, in contrast to the aggravation by caffeic acid (1 or 5 mg/kg, s.c.). Indomethacin (1 or 5 mg/kg, s.c.) prevented PAF-induced gastric bleeding, and it depressed the increase in GMVP in the case of the lower dose. Prostaglandin E2 (50 or 500 micrograms/kg, s.c.) significantly reduced PAF-induced gastric bleeding, but had little effect on PAF-induced increase in GMVP. 1-Benzylimidazole (10 or 50 mg/kg, s.c.) also inhibited PAF-induced gastric bleeding and depressed the increase in GMVP at the higher dose. These results suggest that increased GMVP plays a significant role in producing the gastric damage by PAF. Changes in the mucosal level of cyclooxygenase products, especially thromboxanes, by drugs modifying the arachidonate metabolism would be closely associated with their prevention or aggravation of gastric damage induced by PAF.