["Crohn's disease ileal ulcers and indomethacin-induced jejunal ulceration in the rat tend to occur in the mucosa nearest to the mesentery (mesenteric margin), an area of the bowel wall that has a critical blood supply. Mercuric chloride induces caecal and colonic ulceration in the Brown Norway rat.", 'To examine whether the mesenteric margin is more sensitive to injury by a substance known to be vasculotoxic in the caecum and colon.', 'Brown Norway rats received a single subcutaneous dose of either mercuric chloride 1 mg/kg or saline. The gastrointestinal tract was examined macro- and microscopically for lesions 48 h later. The vascular anatomy of the normal rat colon and caecum was also examined using the carbon ink perfusion technique.', 'Mercuric chloride induced caecal and colonic ulceration preferentially along the mesenteric margin of the bowel wall. Histologically, the lesions showed mucosal necrosis and neutrophil infiltration. There was also extensive vascular degeneration/necrosis with microaneurysm formation and extensive submucosal haemorrhage. Cellular infiltration of the vasculature was not a feature. The caecal and colonic mesenteric margins in control rats were supplied by small end arteries.'] Is the colonic mesenteric margin most susceptible to injury in an experimental model of colonic ulceration?