A very rapid resorption of the amide-type local anaesthetic agents most commonly used in obstetrics results in detectable plasma levels in mother and fetus within 5 to 10 minutes, respectively, after peridural anaesthesia. The various local anaesthetics differ from each other as far as their protein-binding capacity and the resulting ratios of umbilical vein to maternal blood levels are concerned. The feto-maternal ratio is inversely correlated to the degree of protein-binding. Both, the protein-binding capacity and the feto-maternal ratio do not indicate the total amount of the anaesthetic transferred across the placenta. In contrast to measurements of the plasma concentrations only the measurement of the anaesthetic in the fetal tissue would give an exact information about the fetal uptake of local anaesthetics. Several observations hint at the possibility that there are interactions between local anaesthetics and other drugs: A competitive displacement of local anaesthetics from their protein binding sites resulting in an increase in the unbound fraction is suggested. Since the feto-maternal equilibration exists only for the unbound fraction interactions of local anaesthetics with simultaneously administered drugs should be taken into consideration.