Histamine chloramines, derived from the chlorination of histamine by granulocyte-derived oxidants, are potential mediators of intestinal injury and dysfunction in states of atopy or inflammation. We assessed the ability of histamine monochloramine to increase epithelial permeability in rabbit distal small intestine and determined whether the conditions for histamine chloramine formation are favorable in a rabbit model of ileitis. Epithelial permeability, quantified by the blood-to-lumen clearance of ⁵¹Cr-labeled ethylenediaminetetraacetic acid, was enhanced by luminal perfusion with either histamine or histamine monochloramine (10 μM), although the latter was twice as effective (p < 0.05). In a rabbit model of ileitis induced by a luminal solution of acetic acid (200 mM) and casein (10 mg/ml) there was a marked increase in epithelial permeability and in the release into the lumen of histamine, myeloperoxidase, 6-keto-prostaglandin F_1α and protein. These results suggest that the conditions are favorable for histamine chloramine formation and that histamine and histamine chloramine may impair the integrity of the epithelial barrier.