While pharmacology has traditionally been a discipline concerned with therapeutic drugs, many parallels can be drawn between pharmacological considerations in the therapeutic and in the diagnostic realm: adsorption, distribution, potency, selectivity, duration of action, metabolism, excretion, etc. Efforts are underway to develop steroid radiopharmaceuticals as agents to evaluate receptor-positive tumors of the breast, ovary and prostate in terms of prognosis for therapy, and to image steroid-hormone receptors in the brain. In vivo studies with receptor-based radiopharmaceuticals for neuroreceptors in the brain and protein receptors in the liver have led to the evolution of pharmacokinetic models for target tissue uptake and retention. Two general types of models have evolved in connection with the design and use of receptor binding radiopharmaceuticals – equilibrium models and dynamic models. Although there has been a longstanding interest in the preparation of steroid radiopharmaceuticals, it is clear that attempts have been made to correlate structure, specific and non-specific binding in vitro, and uptake selectivity in vivo.