A method of preoperative localization of the site of gastrointestinal bleeding by selective catheterization of the branches of the abdominal aorta has been developed. This was first established experimentally in dogs. By use of the Seldinger percutaneous retrograde femoral-arterial catheterization, the celiac, superior mesenteric, and inferior mesenteric arteries were selectively catheterized. In the dog bleeding points were surgically constructed in various portions of the gastrointestinal tract, including the colon, by means of a catheter placed in a small segmental artery and passed through an enterotomy directly into the lumen of the intestine. The contrast medium was injected automatically and serial films were taken at 1-second intervals for 18 seconds. By varying the diameter of the catheter used, different rates of bleeding were obtained. This technique was successful in demonstrating bleeding points of 0.5 ml per minute and above. This method was then applied to a series of selected patients with