Fluid filtration rate with respect to surface area (Jv/S) from rat mesenteric capillaries was measured before and after platelet-activating factor (PAF) superfusion, a model of acute inflammation. Jv/S increased from a baseline value of 0.015 +/- 0.003 to 0.033 +/- 0.003 micron/s (n = 7 rats, P < 0.01) during PAF superfusion. Measurements of arteriolar pressure with a servo-nulling pressure system suggest that the dominant factor causing the increase in Jv/S during PAF superfusion is increased capillary permeability, rather than an increase in pressure. Jv/S did not increase when PAF superfusion was preceded by administration of the radical-scavenging enzymes catalase and superoxide dismutase, implicating a role for oxygen radicals in the PAF-induced capillary filtration. In another series of studies, three experiments were performed, i.e., PAF superfusion alone or PAF superfusion plus treatment with either a soluble form of sialyl-Lewisx oligosaccharide (SLex) or a nonbinding form of SLex (SLN). These experiments revealed that the increased Jv/S normally elicited by PAF is significantly blunted by administration of soluble SLex but not by the control oligosaccharide SLN. The results of this study invoke a role for oxygen free radicals and selectins (ligands for SLex) in the enhanced capillary fluid filtration elicited by PAF.