Pharmacokinetics is a relatively recent addition to those scientific disciplines which are considered essential for the preclinical evaluation of new drugs. At this time most toxicologists are still torn between the old loyalties for the classical standardized testing procedures and a new approach to safety evaluation in which pharmacokinetic considerations play an important part. Should, I am often asked, a certain drug be tested as admixture to the feed or by gastric gavage? If intubation is the preferred route, should the animals be treated five times weekly or every day? Or should perhaps more than one dose be administered per day? A satisfactory answer to these and similar questions cannot be given without knowing the drug’s pharmacokinetic behavior in the animal species used and the absorption kinetics, blood levels, and excretion of the substance in man. Only then could an experiment be designed in which the exposure of test animals to the new compound be reasonably representative for that occurring in human therapy.