Although intestinal barrier failure after hemorrhage is a well- documented event, the underlying mechanism is poorly understood. The aim of this study, therefore, was to determine whether altered intestinal permeability after hemorrhage is associated with upregulation of local and systemic interleukin-6 (IL-6). To study this, rats underwent laparotomy (i.e., trauma induced) and were bled to and maintained at a mean arterial pressure of 40 mm Hg until 40% of the shed blood volume was returned in the form of Ringer's lactate. The animals were then resuscitated with four times the volume of shed blood with Ringer's lactate over 60 min. At 1.5 h postresuscitation, an in vivo ligated loop of a distal small intestine was formed and the passage of 4-kDa fluorescein isothiocyanate-conjugated dextran (FD4) from the intestinal lumen into the portal vein and carotid artery blood was analyzed by fluorescence spectrometry. Samples from the portal vein and a carotid artery were collected and plasma IL-6 was assayed. Intraepithelial lymphocytes from a distal small intestine were isolated and cultured in vitro for 24 h with or without anti-rat CD3 monoclonal antibody stimulation. IL-6 activity in freshly isolated cells and its release by cultured lymphocytes were determined. Intestinal perfusion and portal blood flow were determined by radioactive microspheres in another set of parallel experiments. The results indicate that lumen-to-blood passage of FD4 through the wall of the small intestine increased significantly at 1.5 h after hemorrhage and resuscitation and was associated with decreased intestinal perfusion and portal blood flow. Plasma IL-6 levels in the portal vein and carotid artery markedly increased at 1.5 h after hemorrhage and resuscitation. In addition, a significant correlation was observed between plasma IL-6 and FD4 concentrations. Higher IL-6 activity in freshly isolated cells was found in hemorrhaged rats. Increased IL-6 release by cultured lymphocytes was also observed either with or without anti-rat CD3 monoclonal antibody stimulation. Thus, the increased intestinal permeability following trauma-hemorrhage and resuscitation appears to be associated with systemic and intestinal IL-6 upregulation.