Sixty-three children with seizure disorders receiving phenobarbital and/or diphenylhydantoin for more than 12 months had liver function tests evaluated. All 56 whose serum anticonvulsant concentrations were in the therapeutic range had elevations of their serum gamma glutamyl transpeptidase activity. Of the 11 who had elevated SGOT and SGPT concentrations initially, six had persistent transaminase abnormalities for more than 20 weeks. Liver tissue from these six patients revealed by light microscopy uniform swelling of the hepatocytes without cell necrosis, inflammation, fibrosis, or disturbance of hepatic architecture. Electron microscopy demonstrated proliferation of the smooth endoplasmic reticulum without other ultrastructural alterations. All six patients were maintained on the same dosages of PB and/or DPH, and their transaminase activities returned to normal within eight to 14 months. The clinical well being of these patients, the transient nature of their SGOT and SGPT elevations, and the absence of specific histopathology suggest that chronic treatment with PB and/or DPH does not result in hepatotoxicity but rather in enzyme induction. The data indicate that liver biopsies are not warranted in such children and that PB and DPH may be continued despite mild elevations of SGOT and SGPT, without concern for hepatic damage.