Temporary small intestinal ischemia was induced by mesenteric arteriolar embolization of degradable starch microspheres in cats. During ischemia, the small intestine received a surface dose of 7 Gy 200 kV x-ray irradiation. One group of animals also had received 7 Gy to the intact abdomen 72 hr earlier. The risk of thrombosis in small intestinal vessels during or after starch microsphere-induced ischemia combined with irradiation was evaluated by monitoring superior mesenteric arterial blood flow, by determination of blood platelets, fibrinogen, and factor VIII consumed across the mesenteric vascular bed, and by histologic examination of small intestinal vessels. Vascular integrity was inferred from intact response to isoproterenol and vasopressin after the combined trauma of ischemia and irradiation. No signs of thrombosis were detected in small intestinal vessels after temporary ischemia and irradiation. Hypoxic radioprotection of the small intestine in the cat can thus be achieved by mesenteric arterial microembolization of degradable starch spheres without evidence of thrombotic complications of significant vascular damage.