Leukocyte/platelet aggregation and aggregate adhesion have been indicated as biomarkers of the severity of tissue injury during inflammation or ischemic reperfusion. The objective of this study is to investigate the mechanisms of the aggregate adhesion and quantitatively evaluate its relationship with microvessel permeability. A combined autologous blood perfusion with single microvessel perfusion technique was employed in rat mesenteric venular microvessels. The aggregate adhesion was induced by systemic application of TNF- plus local application of platelet activating factor (PAF). Changes in permeability were determined by measurements of hydraulic conductivity, Lp, before and after aggregate adhesion in the same individually perfused microvessels. The compositions of the adherent aggregates were identified with fluorescent labeling and confocal imaging. In contrast to leukocyte adhesion as single cells resulting in no increase in microvessel permeability, aggregate adhesion induced prolonged increases in microvessel Lp (6.1 ± 0.9 times the control, n=9) indicated by the initial Lp measurements after 3h blood perfusion, which is distinct from the transient Lp increase caused by PAF-induced endothelial activation in the absence of blood. This prolonged Lp increase was distinguishable from the transient Lp increase caused by PAF-induced direct activation of endothelial cells in the absence of blood. Isoproteronol (ISO) attenuated aggregate adhesion-mediated Lp increases if applied after autologous blood perfusion and prevented the aggregate adhesion if the initial endothelial activation is inhibited by applying ISO prior to PAF administration, but showed less effect on single leukocyte adhesion. This study demonstrated that leukocyte/platelet aggregate adhesion via a mechanism different from that of single leukocyte adhesion caused a prolonged increase in microvessel permeability. Our results also indicate that the initial activation of endothelial cells by PAF plays a crucial role in the initiation of leukocyte/platelet aggregate adhesion.