Monoclonal antibodies, their fragments and low molecular weight oncophilic molecules such as synthetic somatostatin derivatives have been used to deliver radioactivity to target cells for both diagnostic and therapeutic purposes. Clinical studies demonstrated high abilities of the radiolabeled antibodies and peptides for nuclear medicine applications. However, high and persistent localization of radioactivity was observed in the liver or kidney especially when these molecules are labeled with metallic radionuclides, which reduce diagnostic accuracy and compromise therapeutic effectiveness. Thus, radiolabeled antibodies and peptides would become much more useful in both targeted imaging and radiotherapy if the undesirable radioactivity localization can be diminished. As a means to reduce the undesirable radioactivity, interposition of a metabolizable linkage between an antibody and a radiolabel was proposed to generate radiolabeled small molecules of urinary excretion from the parental antibody by enzymatic cleavage of the linkage. In this paper, after indicating the rationale behind the radiopharmaceutical design, a significant role played by the interposition of the metabolizable linkage in altering pharmacokinetics of radiolabeled antibodies is described from a variety of studies so far reported with an emphasis being laid on the importance of radiometabolite-based design of metabolizable linkages.