We investigated the role of free radicals in total occlusion ischemia/reperfusion injury (IRI) of rat small intestine (SI), as measured by changes in macromolecular permeability following pretreatment with free radical blockers or scavengers. We also compared susceptibilities to IRI both along the length of the small intestine, and between tissue layers of its wall. In anaesthetised rats, loops of SI were made ischemic for 1-15 min, followed by reperfusion for 0-10 min. Five min prior to killing, 250 mg/kg fluorescein isothiocyanate bovine serum albumin (FITC-BSA) was injected i.v. Tissue was formalin-fixed, sectioned and examined by fluorescence microscopy. FITC-BSA fluorescence was restricted to discrete focal spots (i.e. blood vessel lumina) in control intestine. In tissue made ischemic, but not reperfused, there was little extravasation of FITC-BSA. In reperfused tissue, there was extensive tracer leakage from vessels, predominantly in the mucosa and submucosa. There was also histological evidence of villus tissue damage. The jejunum was more susceptible to IRI than was the mid small intestine which was more susceptible than the ileum. Superoxide or hydroxyl generation blockers or scavengers (allopurinol, dimethyl sulphoxide or superoxide dismutase) failed to prevent extravasation of FITC-BSA in the mucosa and submucosa of reperfused tissue, indicating that these radicals appear to have no crucial role in total occlusion IRI of rat SI.