The purpose of this work was to assess the effects of DPH on a developed epileptogenic focus in cats with split cerebral hemispheres. The investigations were carried out on 12 cats with a chronic epileptogenic focus produced by means of aluminum method in the right motor area. In all cats the epileptogenic focus was found in EEG. All animals received DPH in daily doses of 8-15 mg/kg. In 2 cats they appeared before beginning of treatment. One of these cats died after 3 days from status epilepticus, the other survived status epilepticus and died after 42 days of DPH administration with signs of intoxication. In 3 cats clinical seizures developed during DPH treatment after 30.84 and 210 days. DPH was given during from 171 to 314 days. Clinical seizures appeared in these cats only sporadically and the animals were sacrificed after completion of investigations. In 7 out of 12 cats clinical seizures failed to develop despite presence of bioelectrically active epileptogenic seizures in the right motor area. Administration of DPH in cats with developed epileptogenic focus failed to prevent clinical seizures. In cats with seizures their control was limited by drug toxicity. In all animals toxic effects were observed although the serum DPH level was in the range 8-20 mug/ml.