Pairs of differently labeled estrogens and androgens (estriol and testosterone; estrone and dihydrotestosterone) were injected into male dogs and baboons and the distribution of the radioactivity in ten tissues and body fluids examined over a period of 120 min by means of a newly available tissue oxidizer. This instrument allows physical separation and determination of mixtures of 3H and 14C radioactivity in the same tissue or fluid. This approach afforded an opportunity to follow simultaneously the fate of two steroids and made comparison of their distribution in animal tissues more reliable. The outstanding finding of the experiments is a significant localization of radioactivity in the pancreas of both animals following estriol or estrone injection. This localization is the more striking, since it greatly exceeded that in such organs as the prostate, testes and skeletal muscle. (Endocrinology90: 1245, 1972)