The bioavailability, serum concentration, biodistribution and elimination data of tracer compounds and their metabolites should be carefully evaluated before they are given to man for scientific, diagnostic or therapeutic purposes. The most important determinants of bioavailability are the extent of absorption and first-pass metabolism. The distribution of tracers to different organs and tissues is determined by the local blood flow and organ-specific barriers, as well as by liposolubility, protein-binding and the binding to tissue structures of the test compound. Moreover, the rates and routes of metabolism and excretion are crucial points in the evaluation of the elimination data of the tracer. The dose of the test substance may also influence markedly the pharmacokinetics of tracer molecules. The factors determining the pharmacokinetic parameters of tracer compounds are summarized.