The developmental neurotoxicity of phenytoin, isotretinoin, and methamphetamine has been reviewed to illustrate effects from both therapeutic and illicit drugs to which people are exposed and which either induce or show the potential for inducing learning disabilities following in utero exposure. In each case both human and experimental animal data are presented and compared where possible. The findings point to several conclusions. First, some drugs in current use induce developmental neurotoxicity, and it cannot safely be assumed that there are not more as yet unidentified. Second, of the types of neurotoxicity induced by drugs, learning disabilities figure prominently. Third, the effects observed are dependent on both the drug's mechanism of action and the period of brain development during which exposure occurs. Fourth, with the exception of CNS teratogens, it is not yet possible to predict which periods of brain development are the most vulnerable for the induction of learning disabilities, as seen by the different patterns of critical periods for phenytoin and isotretinoin compared to methamphetamine. Fifth, as seen with isotretinoin, existing drugs that cause developmental neurotoxicity are not the only problem; new drugs with such effects are still being introduced. Sixth, only a small fraction of the drugs currently in use have ever been examined for developmental neurotoxicity; hence, the full scope of the problem cannot even be accurately estimated based on current information. It is concluded that prevention of new cases caused by drugs such as isotretinoin should be a high priority for future regulatory action.