A number of animal models have been used to evaluate the developmental effects of exposure to toxicants, and several of these are exemplified by the standard protocols used by regulatory agencies. Others have been developed to evaluate the effects of toxicants on particular target organs or organ systems. Animal models and principles for the interpretation of data from studies involving prenatal exposure are much further advanced than those involving postnatal exposure. In those experimental animal studies that assess the effects of prenatal exposure, the developing organism is exposed secondarily via the maternal organism which intervenes in the absorption, metabolism and distribution to the embryo/fetal compartment. In postatal exposure studies, exposure of the developing offspring may also be indirect via maternal milk or direct by dosing of pups. In addition, exposure may occur unintentionally via contaminants on maternal fur or skin, in excretory products following exposure, and/or by access to dosed food or drinking water before weaning.