Liposomes with different charge, size and lipid composition were injected i.v. into pregnant rats and rabbits and their uptake and transport across the placenta were studied. Results show that the placenta is very efficient in removing liposomes from the maternal circulation and that it takes up more liposomes per gram of tissue than the liver. Liposomes are degraded intracellularly in the placenta and the entrapped material is then transported across to the fetus as free molecules. Under the experimental conditions described in this article, no intact liposomes were found to be transported across the placenta. In comparison to the i.v. injection, infusion of liposomes into pregnant rabbits had greater effect on the localization of liposomal drug in the placenta and fetal blood. In summary, these results show that small unilamellar vesicles enhance delivery of drugs and nutrients to the placenta. They also suggest the possible danger of toxicity to the fetus if the expectant female is undergoing liposomal drug treatment.