Since inhalation of CO2 prevents electrically induced cortical seizures in cats, a study on primates was undertaken. Rhesus monkeys were chosen because of ease of extrapolation to man. Monkeys, fitted with bilateral screw-in electrodes and immobilized with dihydro-beta-erythroidin, were artificially respirated. The animals, ventilated with various concns. of CO2-O2 for different intervals of time, were stimulated with threshold and super-threshold voltages. It was found that 15% CO2 administered for 3 min. would not prevent seizures, though longer periods of inhalation suppressed seizures. Concns. over 20% prevented seizures.