The introduction of substituents at or adacent to the major sites of oxidation will influence the metabolic profiles observed. This chapter focuses on the metabolic transformations of the major estrogens in mammalian species, including man and identifies the effects that the incorporation of specific radionuclide containing groups may have upon those pathways. The transformations have important implications for the labeled derivatives. The majority of the newer radiolabeled estradiol derivatives have incorporated the radionuclide on the D-ring of the steroid nucleus. This has been the result of structure-activity relationship studies that have demonstrated that the introduction of certain substituents at the 16α- and or 17α-position can be accomplished without seriously diminishing the estrogenic activity of the product. The chapter examines the two classes of radiolabeled estrogens, the 16α-halo-estrogens and the 17α- substituted derivatives, with respect to the potential effects of those substituents upon metabolism. The 16α-halogenated estradiol derivatives will most closely resemble the corresponding estriols in their metabolic transformation.