The aim of this study was to assess the extent of collateral blood flow provided by the celiac and inferior mesenteric arteries to the intestines during total occlusion of the superior mesenteric artery (SMA). In anesthetized cats, blood flow to the pancreas, duodenum, jejunum, ileum, and colon was measured with radioactive micropheres (15 microns in diameter) before and during occlusion of the SMA. Superior mesenteric artery occlusion significantly decreased (by 63%) blood flow to the head of the pancreas. Flow to the neck and tail of the pancreas was not altered. Blood flow to the proximal and distal duodenum was significantly reduced by 35% and 61%, respectively. Along the entire jejunum and ileum, SMA occlusion markedly decreased blood flow by an average of 71%. In the proximal colon, blood flow decreased by 63%, whereas flow to the middle and distal colon was not affected by SMA occlusion. Reduction in total wall blood flow to the small and large intestines was largely due to a marked reduction in mucosa/submucosa blood flow; muscularis/serosa flow was not affected. The results of this study suggest that total occlusion of the SMA does not compromise blood flow to the neck and tail of the pancreas and middle and distal colon (tissues that are normally perfused with blood from either the celiac or inferior mesenteric arteries). Perfusion through collaterals maintains flow to the head of the pancreas and gut (from duodenum to proximal colon) to within 30%-65% of control (preocclusion) flow. An important new observation of this study is that collateral blood vessels are much more effective in preventing ischemia in the muscularis/serosa than in the mucosa/submucosa after SMA occlusion.