Insulin labeled with 131I or 125I was injected into the femoral or hepatic portal vein of albino rats, and autoradiographs of tissues and organs were prepared. Diffuse radioactivity, which varied widely in intensity from one region to another, occurred in the connective tissues. Localization of radioactive material in elastic tissue was readily detectable 1 and 15 min after injection. Radioactive material accumulated in elastic tissue of larynx, trachea and bronchi, splenic capsule and trabeculae, atrial endocardium, elastic cartilage of the xiphoid process and the tunica intima of large veins. Radioactive material occurred through full sections of the wall of the pulmonary artery and its major branches, and in the outer third of the elastic media of the aorta. Control tissues prepared after injection of monoiodotyrosine-131I, Na131I and enzymatically degraded insulin-125I gave negative results. In confirmation of previous findings, concentrations of radioactive material in the proximal tubules of the kidney, and in, on, or near phagocytic endothelial cells of the liver (Kupffer cells) were observed. These results indicate a distinct affinity of elastic tissue for insulin or its breakdown products, and a non-uniform distribution of this hormone or its breakdown products in the connective tissues generally. (Endocrinology74: 914, 1964)