The ability of cortisone to modify EEG and seizure activity was investigated in the baboon, Papio papio. Acute intramuscular doses (0.5-4 mg/kg) caused a dose-dependent increase in seizure response to a flashing light stimulus. This increase in seizure response was apparent in both seizure duration and the spread of convulsive activity. Along with enhancement of seizures, cortisone was found to cause marked changes in the EEG, ranging from the appearance of interictal paroxysmal activity to alterations in spectral characteristics of the wave forms. Increases in slow waves appeared concomitant with a decrease in fast activity in the 18-25 c/sec range. Since previous studies have indicated that seizure proclivity in the Papio papio is maximal at the time of the day when cortisol excretion rates peak, these findings lend further evidence to the idea that corticosteroids may be involved in the thythmic variation of seizure activity in the baboon.