Studies of congenital abnormalities in mothers treated for epilepsy have implicated trimethadione, phenytoin. valproic acid and carbamazepine as potential teratogens. The teratogenicity of drugs such as phenytoin is suspected to be mediated not by the parent compound but by toxic intermediate metabolites. Oxidative intermediates (epoxides) eliminated in a reaction catalyzed by epoxide hydrolase are believed to be the primary teratogenic agents of phenytoin and also carbamazepine. Thus there might be a relationship between epoxide hydrolase activity in amniocytes and the risk of anomalies associated with fetal hydantoin syndrome.