Neurobehavioral assessment has become an important tool for evaluating the con­ sequences o f intrauterine exposure to chemical agents routinely encountered in the environment as well as substances, including alcohol and drugs, used by the mother during pregnancy (Riley & Vorhees, 1986). At first, it was assumed that the placenta acts as an effective barrier preventing toxic substances in the mater­ nal blood stream from reaching the fetus. It is now understood that most toxic substances cross the placenta, the exceptions being those that are nonlipid solu­ ble, ionize at tissue pH, or have a molecular weight greater than 600 (Coyle, Wayner, & Singer, 1976). Early experimental research revealed the role o f irradi­ ation, viral and bacterial infection, and malnutrition in producing fetal malfor­ mations previously thought to be genetically caused (Hutchings, 1978). The thalidomide tragedy (Tuchmann-Duplessis, 1975) and the identification o f fetal alcohol syndrome (Jones & Smith, 1973) attracted greater attention to the field and made clear that a variety o f chemical agents could be harmful to humans. More recently, man-made environmental substances have come under scientific scrutiny to determine their potential teratogenicity and whether they are harmful at dosages ordinarily ingested or absorbed by humans. Findings such as these have stimulated a new research area concerned with the impact o f toxic sub­ stances on human development and functioning.