Plasma levels of gamma-glutamyl transpeptidase were measured prospectively in 75 epileptic patients treated with one of the following drugs: phenobarbitone (n = 20), phenytoin (n = 23), carbamazepine (n = 18) or sodium valproate (n = 14). A significant increase in gamma-glutamyl transpeptidase levels was observed from the 7th day of treatment onward. The increase was particularly pronounced in the phenytoin group, with mean and maximum values of 104.10 and 215 mU/ml respectively, followed by the phenobarbitone group (mean 68.15, max. 124 mU/ml), the carbamazepine group (mean 49.83, max. 100 mU/ml) and the sodium valproate group (mean 35.42, max. 70 mU/ml). This was due to enzyme induction being highest with phenytoin and phenobarbitone which are strongly liposoluble and have prolonged half-life. Any epileptic patient with plasma gamma-glutamyl transpeptidase levels higher than those found in this study may be suspected of having viral or alcoholic hepatitis.