Rapid intravenous administration of chlorpromazine (2-10 mg/kg) to 13 epileptic and 5 normal monkeys produced prompt profound behavioral changes characterized by hypokinesia, flaccidity, diminished responsiveness, reduced aggressiveness, lethargy and somnolence. High amplitude slow wave activity (2-5/second) became dominant in the eeg after intravenous injection of chlorpromazine; epileptic monkeys showed, in addition, increased high amplitude spike and sharp wave forms. Epileptic monkeys under the acute influence of chlorpromazine retained their clinical convulsive responsiveness to intramuscular Metrazol (16 mg/ kg or less). Clinical epileptic response to vigorous prodding was not significantly changed. Normal control monkeys under the acute influence of chlorpromazine did not exhibit clinical convulsions and remained refractory to treatment with intramuscular Metrazol (32 mg/kg) and to vigorous prodding stimulation.