Ischemia-reperfusion (IR) injury is a major insult to postcapillary venules. We hypothesized that IR increases postcapillary venular hydraulic conductivity and that IR-mediated changes in hydraulic conductivity result from temporally and mechanistically separate processes. A micro-cannulation technique was used to determine hydraulic conductivity (L_p) in rat mesenteric post-capillary venules serially throughout ischemia (45 min) and reperfusion (5 hrs) induced by superior mesenteric artery occlusion and release. Mesenteric IR resulted in a biphasic increase in L_p. White blood cell adhesion (WBC) slowly increased with maximal adhesion corresponding to the second peak (p<0.005). After IR, tissue was harvested for RT-PCR analysis of ICAM-1, E-selectin, and P-selectin mRNA. ICAM-1 mRNA in the gut showed the most significant upregulation. Quantitative real time PCR revealed that ICAM-1 mRNA was upregulated 60-fold in the gut. An ICAM-1 antibody was therefore used to determine the effect of WBC adhesion on L_p during IR. ICAM-1 inhibition attenuated Lp during the first peak and completely blocked the second peak (p<0.005). When rats were fed a tungsten diet to inhibit xanthine oxidase and then underwent IR, L_p was dramatically attenuated during the first peak and mildly decreased the second peak (p<0.005). Inhibition of xanthine oxidase by oxypurinol decreased L_p during IR by over 60% (p<0.002). Tempol, a superoxide dismutase mimetic, decreased Lp during IR by over 30% (p<0.01). We conclude that IR induces a biphasic increase in post-capillary hydraulic conductivity. ROS impact both the first transient peak and the sustained second peak. However, the second peak is also dependent on WBC-endothelial cell adhesion. These serial measurements of postcapillary hydraulic conductivity may lead the way for optimal timing of pharmaceutical therapies in IR injury.