AIM:To investigate the role of peripheral blood leukocytes and polymorphonuclear neutrophils (PMNs) on intestinal injury following mesenteric ischemia/reperfusion (IR) in rats. METHODS: Twenty adult, male Sprague-Dawley rats, weighing 200-230 g, were randomly divided into two groups. The control group (CON) consisting of 10 rats was subjected to laparotomy and separation of superior mesenteric artery (SMA) only. The ischemia/reperfusion (IR) group consisting of 10 rats, was subjected to laparotomy, followed by occlusion of the superior mesenteric artery (SMA) by an atraumatic microvascular clamp for 30 min. At the end of ischemic period in IR, the microvascular clamp was removed and the intestinal segment was reperfused for 60 min. The pathological changes of the ileal mucosal tissue were evaluated. The apoptosis of intestinal mucosal epithelial cells was examined by terminal deoxylnucleotidy-l transferase mediated-dUTP nick end-labeling (TUNEL). The enzymatic activity of casapse-3 in mucosal cells was determined using a colorimetric assay. The percentages of apoptotic peripheral blood leukocytes and PMNs were measured by flow cytometry using Annexin-V/PI double staining assay. The numbers of peripheral blood leukocytes in each animal was measured at baseline, 30 min of ischemia, and 30 min and 60 min of reperfusion. RESULTS: (1) Compared to CON group animals, the most severe mucosal injury was observed in IR group under optical microscope. (2) The number of apoptotic mucosal epithelia cells and enzymatic activity of caspase-3 were significantly higher in IR than those in CON group (P