Mucosal lesions were produced in feline small intestine by evoking a simulated intestinal shock (local hypotension at 30 mm Hg and stimulation of regional sympathetic vasoconstrictor nerves at 6 Hz for 2 h). The degree of mucosal damage was correlated to the level of intestinal blood flow. Microscopically characteristic lesions developed regularly in the small intestinal mucosa when intestinal blood flow was reduced below 12 ml/min X 100 g during the regional shock. The mucosal damage was graded histologically. No difference was found between untreated controls and cats in which the intestinal lumen was perfused with nitrogenated saline. Perfusion with oxygenated saline and i.v. injections of methylprednisolone on the other hand, prevented almost completely the development of the lesions. Albumin, activated charcoal and aprotinin instilled into the intestinal lumen reduced to some extent the mucosal damage. The obtained data support the view that hypoxia is the key factor in the pathogenesis of the mucosal lesions. However, epithelial and intraluminal enzymes are probably important contributing factors.