The effect of the oral administration of 30 mg. of methylphenidate daily on serum levels of phenobarbital, diphenylhydantoin, and primidone was evaluated in 11 patients. One group of patients was assigned to an 18 week treatment period of 6 weeks each for control, methylphenidate, and control; another group was studied in similar fashion for methylphenidate, control, and methylphenidate. Plasma anticonvulsant levels were determined at weekly intervals. No significant differences in plasma anticonvulsant levels were noted during methylphenidate administration.