Any drug given to a pregnant woman must be considered as possibly harmful to the fetus, since all drugs administered to the mother cross the placental membrane, although at different rates. Important physiological changes occur in pregnancy, which may influence the kinetics of drugs. Differences in gastrointestinal function are likely to alter drug absorption rates in the stomach or gut. Ventilatory alterations modify pulmonary drug absorption or elimination. Important changes in haemodynamics and alterations in body water compartments influence drug distribution and elimination. Renal drug elimination is generally enhanced, whereas hepatic drug elimination may be modified in different ways.Computerised pharmacokinetic models representing the compartmental aspects of the fetalmaternal unit and fetal-maternal drug interrelationships may be used to predict kinetic consequences of fetal drug exposure. Such information may be a useful guide for the clinical use of drugs during pregnancy, particularly for treatment of fetal disease.