The hamster cheek pouch was used to determine the role of platelet activating factor (PAF) and nitric oxide (NO) in leukocyte adhesion and microvascular leakage of FITC-dextran following systemic administration of endotoxin. Endotoxin (5 mg/kg, i.v.) or vehicle was administered to the hamster 15 min after systemic pretreatment with either a PAF antagonist (Abbott-87648, .1 mg/kg, i.v.) or a nitric oxide synthase inhibitor (L-NAME, 30 mg/kg, i.v.), or 60 min after dexamethasone pretreatment (4 mg/kg, i.p.). Endotoxin alone caused rapid leakage of FITC-dextran from the vascular compartment into the tissue. This leakage was not preceded by either increased leukocyte rolling or adhesion in postcapillary venules. Pretreatment with the PAF antagonist, as well as with dexamethasone, completely blocked endotoxin-induced leakage. L-NAME also blocked endotoxin-induced leakage; however, more than 50% of the hamsters treated with L-NAME died within 2 h after endotoxin administration. These results suggest that PAF and NO are important mediators of microvascular leakage during endotoxemia and that their actions are not dependent on leukocyte adhesion to postcapillary venular endothelium.