Mucosal injury is an immediate event following revascularization of small intestinal grafts in the context of transplantation (SBTx). The generation of oxygen free radicals (OFR) and tissue infiltration by activated neutrophils are consequences of ischemia and reperfusion and known causative factors of tissue injury; to delineate their role in the reperfusion injury occurring after cold preservation of the intestine and subsequent transplantation was the aim of this study. Prior to orthotopic SBTx in Sprague-Dawley rats, grafts were stored in cold (4 degrees C) Ringer's lactate solution for 1 (n=6), 2 (n=7), and 4 hr (n=7). Small bowel biopsy specimens were obtained before harvesting, at the end of the (cold) ischemic period and immediately before unclamping (i.e., before revascularization) and 30, 60, 120 min, and 24 hr after transplantation to evaluate tissue injury by histology, OFR production, (measured by luminol-enhanced chemiluminescence [LCL]), and the degree of neutrophil infiltration by myeloperoxidase staining. Reperfusion of the graft significantly worsened the histologically graded mucosal injury compared with that seen before unclamping. However, 24 hr after engraftment, mucosal morphology was restored almost completely. OFR production increased significantly during the early phases of reperfusion (30, 60, and 120 min) and returned to control values after 24 hr. Reperfusion of the graft was associated with a marked increase in the number of mucosal neutrophils. The present study indicates that OFR production and neutrophilic infiltration commence and progressively increase with graft reperfusion. These changes parallel the mucosal injury. Ischemic intervals of 4 hr were not associated with a statistically significant greater ischemic-injury patterns compared with 1- and 2-hr intervals. The profound changes associated with reperfusion probably overshadow the minor, yet likely, progressive injury patterns associated with longer ischemia times.