Receptors are proteins that have specific binding affinity for substances that produce a physiological event in the body. Receptor-binding radiotracers are being used increasingly to study the function of receptors in health and disease. This review summarizes the proceedings of a symposium on research in the development of receptor-binding radiopharmaceuticals. The key phases in this research include: selection of the receptor system and ligand; synthesis of radiolabeled ligand; validation in animal models; and clinical application. Current research involves a variety of biological systems, such as butyrophenone neuroleptics for dopamine receptors and steroidal estrogens for the estrogen receptor. In the future, it is believed that receptor-binding radiopharmaceuticals will be useful, not only to validate receptor systems in vivo, but also to aid in the diagnosis and therapy of human diseases.