This series of two review articles deals with general aspects of the mechanisms which govern absorption and distribution of drugs within the body, and the kinetics of drug metabolism and excretion. Emphasis is given to the verification of pharmacokinetic principles in domestic animals, and the clinical applications which proceed therefrom. In the first part, the concept of the two compartment and three compartment models for drug disposition are introduced and developed to demonstrate the mathematical models which may be derived to describe the kinetics of drug distribution and elimination. In the second part bioavailability and drug dosage will be considered.