The evidence of the transplacental transfer of most of the drugs administered to the mother has changed the old concept of the invulnerability of the fetus due to the placental barrier protection. The commonly used drugs act on the fetus by means of two different mechanisms: directly after the placental transfer, and indirectly influencing the utero-placental circulation and maternal homeostasis. Fetal effects can be several and they depend on the type of the drug, dosage, route of administration, gestational age in relation to the fetal ontogenesis, and on the amount of drug which reaches the fetus. For example, some drugs are redistributed in the maternal compartment; others remain in the fetal compartment by virtue of their chemical-physical characteristics and/or altered fetal homeostasis, i.e., fetal acidosis results in ionisation of the loco-regional anesthetics, which cannot leave the fetus, and therefore are entrapped in the fetal compartment. The complexity of the fetal-maternal pharmacology has implemented research on the animals, and epidemiologic and clinic studies. In spite of the research performed in the last years, the long-term perinatal effects are still to be clarified for many drugs administered to the mother.