Diagnostic radiopharmaceuticals are administered to provide information rather than to induce pharmacologic response. Therefore, one cannot translate pharmacologic principles established for stable compounds directly to radioactive compounds. If radioactive materials are to be used safely and effectively, certain principles must be followed. Radiation to the patient must be minimized. Radiation detection instruments must be sensitive enough to yield statistically significant data with small doses of radiopharmaceuticals. Radionuclides with a rapid rate of physical decay are preferred, although, if a radioactive material is excreted rapidly and completely, the exposure to mdiation may be low despite long physical half‐life. It is important to use radioactive isotopes with no beta raciiations, or with low energy beta radiations. Because scintillation detectors which effectively measure only gamma radiation are most often used, beta radiation contributes to radiation dosage but provides no data. Special pharmaceutical problems associated with radiopharmaceuticals are: (1) problems of carrier‐free state, ie., those arising because of the extremely small chemical quantities; (2) problems of self‐decomposition, produced by the effect of radiation on the compound itself or on its solvent; (3) problems resulting from isotope effect, i.e., differences in reaction rates that sometimes result from differences in atomic weight of isotopes; and (4) problems resulting from chemical differences betwecn labeled and naturally occurring compounds.