This symposium presentation outlines the activity of the Food and Drug Administration (FDA) in regulating the use during pregnancy and lactation of drugs which affect the fetus and neonate. When presented with an unexpected adverse effect of a drug or of a class of drugs, the FDA can take several steps. These steps include ordering total removal from the market, restricting use, issuing warning statements and recommendations, providing information through official publications of the FDA, and other educational efforts. Identifying subtle adverse drug reactions is a difficult problem. Epidemiologic, prospective, retrospective, and animal studies are used. Since 1962, the FDA has required that safety studies be conducted in animals before a new drug is introduced into human subjects. Before any drug may be administered to subjects in whom there is a potential for pregnancy, teratology studies in at least two species of animals must be completed. While reproduction studies are designed to disclose teratogenic, direct toxic, gross behavioral and various other effects of the drug on the fetus or the young, they are not directed toward the detection of subtle neurological defects that may not become manifest until some time after birth. Frequently, replication of results of animal test studies is not easily obtained. (Author/RH)