The authors attempt to evaluate the effects of dilantin Na on the seizure incidence, conduct, and intelligence and performance ratings of 29 chronic idiopathic and 44 symptomatic epileptics. The drug was administered over a period of 12-18 mos. About 65% of the patients experienced a marked reduction in number of seizures. One-half the patients exhibited an appreciable improvement. Although this occurred in those who had a marked diminution in the number of seizures, a proportionate group who also had fewer attacks showed no such improvement. Many patients, apparently as a result of having fewer spells, gave the impression of being brighter and more intelligent than they were before dilantin was given them. Actually, however, there was, in general, little . significant rise in their intelligence ratings. A more significant improvement was shown in the performance ratings of a small group, suggesting that, as a result of its anti-convulsant action, dilantin has a more beneficial effect on performance capacity than on intelligence. The drug was about as effective in the idiopathic as in the symptomatic groups of epileptics.