In induced bleeding experiments on dogs, 99mTc-sulfur colloid was a suitable agent for detecting the bleeding site in the small intestine, providing that the site was distant from the liver and spleen. Bleeding sites were detectable at rates as low as 0.1 ml/min. When induced in the sigmoid or descending colon, the site was demonstrated by scintigraphy with 99mTc-sulfur colloid. Unsatisfactory images were obtained in the esophagus and stomach, however, when 131I-ortho-iodohippurate or 99mTc-DTPA was used.