This article is an attempt to present issues associated with the principles of GLP system harmonization, particularly in relation to pharmacokinetic (PK) studies at a global scale. Complete harmonization of GLP principles requires unification at several levels: inside registration authorities, between key registration authorities, within the framework of procedures regulating preclinical and clinical phases of the drug-development process and within the framework of procedures regarding GLP principles used in PK analyses and analyses of residuals of veterinary drugs. This large number of discrepancies indicates that total harmonization of rules on this issue will be very difficult and will require close cooperation between institutions responsible for legislative processes and control of GLP principles during PK analysis.