Abstract : Acute ligation of the common colic and caudal mesenteric artery in the dog led to ischaemic colitis, whereas the acute ligation of the common colic artery and gradual occlusion of the caudal mesenteric artery by an ameroid plastic device did not lead to ischaemic disease. These findings suggest that the collateral blood vessels do not open up rapidly enough to prevent ischaemic change when the two major arteries are acutely ligated.