The effect of various doses (40--160 mg/kg) of isatin (2,3-dioxoindoline) on electroencephalographic manifestations of vigilance, groups of rhythmic discharges (episodes) and the incidence of epileptiform pathological activities was studied in repeated chronic experiments on rats with implanted cortical and subcortical electrodes. After all the given doses of isatin the proportion of slow-wave sleep diminished and its onset was delayed (especially REM sleep) and the amount of quiet wakefulness increased. There were more spontaneous rhythmic episodes and their rhythm slowed down. In some cases large doses of isatin produced short groups of discharges without any motor signs; sometimes the episodes changed to epileptic activity composed of spike-wave complexes (2--3 Hz), accompanied by clonic jerks.