In doses of 160 and 80 mg/kg, isatin (2,3-dioxoindoline) significantly reduced the total incidence of audiogenic epileptic seizures in rats highly sensitive to an acoustic epileptogenic stimulus. The number of severest forms of seizure (running, clonic convulsions) was higher than in the control tests, however. The acoustic epileptogenic stimuls was applied one hour after the i.p. injection of isatin. At that time some postural reflexes were still inhibited after 160 mg isatin/kg, while after smaller doses they were already normal again. One hour after administering isatin there were marked changes in the electroencephalogram, the chief ones being an increase in rhythmic episodic activity against a desynchronization background and a decrease in slow wave sleep activity.