Diabetes mellitus is of obscure etiology, often presenting profound metabolic derangement with a sparsity of morphopathologic change. Insulin reverses many of the metabolic changes, yet its mechanism of action is poorly understood; furthermore, in most patients with diabetes mellitus, the fundamental causes of the insulin insufficiency are not known. An important approach to this challenging problem is the study -of insulin metabolism. Such a study has been hampered by unsatisfactory assay techniques, and, as a-result, very little is known even of the fate of insulin. The use of labeled hormone greatly facilitates such a study, and, in this report, the validity of such an approach is considered, and distribution and degradation studies using I'8l-labeled insulin are described.