Serum gamma-glutamyl transpeptidase (gamma-GT) activity was significantly (p less than 0.001) elevated in 85 (49.7%) out of 171 epileptics receiving anticonvulsant therapy. The effects of phenytoin, phenobarbital, carbamazepine, and valproate sodium on gamma-GT activity were analyzed statistically. Among these anticonvulsants, only phenytoin seemed to be responsible for the elevation of gamma-GT activity, since its daily dose and serum level were significantly (p less than 0.05) related to the gamma-GT value. Fifteen out of 17 epileptics with abnormal serum glutamic pyruvic and/or oxaloacetic transaminase levels also showed considerable elevations of gamma-GT values. The possible relationships between the enzyme induction of liver microsomes or chronic hepatic toxicity and the elevation of serum gamma-GT activity in epileptics under treatment are also discussed.