Despite the widespread use of phenytoin in the treatment of epilepsy, few studies have examined its effects on cognitive functioning. We report here the results of an investigation into the effects of phenytoin on the performance of non-epileptic volunteers, on a series of specially designed psychological tests. Subjects received placebo or phenytoin 300 mg per day for two weeks in a double-blind crossover design and measures were taken of phenytoin serum levels. The results demonstrate a deleterious effect of phenytoin upon several important aspects of cognition, including memory, concentration, mental and motor speed. The implications of our findings for the treatment of patients with epilepsy are discussed.