The action of ethosuximide, valproate and clonazepam against pentylenetetrazol-induced epileptic EEG phenomena was studied in acute experiments in rats with intercollicular brainstem transection. Ethosuximide lost its action against both rhythmic metrazol activity (model of human absences) and EEG seizures. On the contrary, the action of valproate and clonazepam in cerveau isolé rats was the same as in intact animals. The site of anticonvulsant action of ethosuximide may be localized in hindbrain structures, whereas the actions of both valproate and clonazepam may be demonstrated even if hindbrain structures had been eliminated.