Psychometric, adaptive ability, motor proficiency, and sensory discrimination measures were administered to adult seizure patients within 36 hours of blood serum determinations of diphenylhydantoin, phenobarbital, and primidone. Thirty-five subjects in whom at least one of the three drug levels fell within stated μg per milliliter toxicity ranges were compared with 28 subjects of comparable age and IQ who were identified by the same procedure to be nontoxic. The toxic group consistently obtained poorer mean test scores, and significant intergroup differences were found on several measures of attention and concentration and on tests of motor coordination and static tremor.