PURPOSE: The purpose of this animal and clinical study was to compare intra-arterial(IA) scintigraphy withangiography in the localization of gastrointestinal (GI) bleeding. MATERIALS AND METHODS: After sedation withintramuscularly administered ketamine, lower GI bleeding was induced in ten rabbits. Using inguinal cut-down, anarterial femoral 3F catheter was placed in the proximal mesenteric artery. Following abdominal incision to exposethe bowel, lower GI bleeding was caused by incising the antimesenteric border of the small bowel wall. Initialangiography was performed, and this was followed by Tc-99m pertechnetate IA scintigarphy. Tc-99m RBC IAscintigraphy involved two patients who had undergone selective mesenteric arterial catheterization for theevaluation of acute lower GI bleeding. RESULTS: Ten rabbits, bleeding at a mean rate of 0.7g/min, were studied.IA scintigraphy was superior to angiography in four cases and equal in six. The sensitivity of angiography was40%(4/10), and IA scintigraphy 80%(8/10). In one patient, Tc-99m RBC was administered directly into the superiormesenteric artery and ulcer bleeding in the transverse colon was identified. Prior to conventional angiography,the bleeding had been occult. In a second patient, in whom angiography had revealed a hypervascular mass,selective injection of Tc-99m RBC into the superior mesenteric artery revealed tumor(leiomyoma) bleeding in thejejunum. CONCLUSION: Selective IA scintigraphy was valuable for detecting intestinal bleeding, occult duringconventional studies and may be useful for detecting acute bleeding at the time of negative angiography.