Breakdown of the blood-retinal barrier occurs in inflammatory conditions and in ischemic retinal diseases such as diabetic retinopathy. Platelet-activating factor (PAF) is a potent inflammatory mediator which increases vascular permeability. The purpose of this study was to determine if intravitreally-injected PAF would cause breakdown of the blood-retinal barrier and, if so, by what mechanism. Fluorescein angiography was performed before and at 0.5, 1, 2, 3 and 4 hr after PAF injection into the vitreous cavity of rabbit eyes and the eyes were enucleated immediately for light and electron microscopy. Slow flowing thrombi were observed in all PAF-injected eyes. Complete vascular occlusion was observed in 10 of 16 eyes after 3 and 4 hr. There was no fluorescein leakage in any of eyes before or at 0.5 or 1 hr after PAF injection. Fourteen of 20 eyes had fluorescein leakage at 2, 3 and 4 hr after PAF injection. The extent of fluorescein leakage correlated with the degree of polymorphonuclear leukocyte (PMN) margination, disruption of the endothelial cell layer, infiltration into vascular walls and migration into the vitreous cavity. PMNs appeared to migrate by both intercellular and transcellular routes across the endothelium. Pretreatment of rabbits with a PAF inhibitor, BN52021, prevented most of the abnormal findings.