Graded small intestinal ischemia was induced in cats by partial occlusion of the superior mesenteric artery. Specimens from the small intestine were studied at the end of the hypotensive period, 10 min and 60 min posthypotensively . Specimens obtained late during hypotension revealed characteristic mucosal damage in all cases. However, the intestinal mucosal damage was more pronounced in the specimens taken posthypotensively . This aggravation was prevented if the cats were given superoxide dismutase (SOD) during the hypotensive period. Further, the intestinal tissue concentration of total glutathione was reduced posthypotensively . It is suggested that the posthypotensive aggravation of the intestinal mucosal damage is caused by generation of cytotoxic oxygen free radicals. Data favouring the suggestion that the glutathione-glutathione peroxidase system is an important generator of free radicals in intestinal ischemia is presented.