Pregnancy is a specific dynamic state, and the potential usefulness of caring for a disorder in the fetus or the mother is now well established. Previously, pregnant women have been excluded from clinical trials, therefore only a few studies concerning evaluation of the pregestational metabolism or transplacental transfer (TPT) of drugs exist. Questions regarding the TPT of drugs are extensive and complex. For example, does TPT occur at a given gestational age, in the context of a particular type of pathology or when a drug is administered by a certain dosage regimen? If this is the case, what is the rapidity of penetration of the products of conception by the drug (bearing in mind its physicochemical characteristics)? Need harmful adverse effects on the child be feared? Is such penetration desirable, of no consequence, or dangerous? Does the possibility exist of accumulation in the placenta, fetal tissue or amniotic fluid? Should such findings modify the therapeutic regimens of drugs given to expectant mothers? Exchange mechanisms are complicated and models developed in vitro only partially reflect the actual equilibria that exist between mother and fetus. These include: (i) the perfused cotyledon model, which while simple, elegant and inexpensive, offers only a localised, restricted and fixed view of pregnancy; (ii) isolated anatomical fractions that are informative, but which straddle the border between physiology and pharmacology; and (iii) the necessary study, using microsomes, of placental metabolic capacity (enzyme cartography). In vivo study of TPT is based upon various multicompartmental pharmacokinetic models, some of which have been relatively validated in animals. The simplest indicator for the in vivo evaluation of TPT of a drug in the human species is determination of a feto-maternal blood concentration ratio (usually performed at the time of placental separation). However, the usefulness and limitations of this parameter are controversial, and it would seem preferable to associate it with a pharmacokinetic profile of variations in blood concentrations established in the mother. Furthermore, any extrapolation of a single result to fetal and adjacent tissues must be done with the greatest caution. Although, no drug should be used in pregnancy unless there is a clear therapeutic indication, study of the TPT of therapeutically useful agents is essential to the understanding of their metabolism and is a prerequisite to the safe use of medications during pregnancy.