Antiepileptics can impair the performance of subjects in both acute and subacute doses. The effects of phenobarbital and its derivatives and benzodiazepines are stronger in this respect than that of phenytoin, ethosuccimides and valproic acid. Carbamazepine, in contrast, has rather a stimulatory action. Impaired cognition, psychomotor performance and emotions must also be expected under therapeutic conditions. Attention is drawn to various factors that may influence the results of tests, including the dose and blood concentration, interval between application and test, initial performance of the probands or patients, levels of vigilance and personality structure. Lack of compliance is particularly important since adaptation and readaptation processes that can be dangerous take place in these patients. The careful exploration of side-effects on the patient may have a decisive influence on practicability and cost if the relationship between patient and doctor is good.