Pharmacokinetics studies are performed to clarify the absorption, distribution, metabolism, and excretion of drug candidates. The data are not only useful for the planning of animal toxicity studies and pharmacological studies but also required for establishing the efficacy and safety in humans by indicating the pharmacokinetically appropriate drug formulation and drug usage. Identification of the major metabolic enzymes related to the major metabolism of a drug is also essential for predicting pharmacokinetic drug interactions. Use of human tissues in drug metabolism studies becomes more and more important for rational and efficient drug development. It is necessary to establish an organization in Japan for supplying human tissues originating from the Japanese population.