Overventilation is particularly effective in precipitating petit mal seizures, as has been pointed out by Gibbs, Lennox and Gibbs.1 Only rarely in their experience has any other form of epileptic seizure resulted from vigorous overbreathing. From this one can conclude that rapid induction of respiratory alkalosis gives rise to a physicochemical state of the fluids bathing the nerve cells in the brain which is favorable for the appearance of the petit mal attack. One cannot say, however, whether this physicochemical state in patients with petit mal epilepsy is different from that obtaining in nonepileptic persons when they overbreathe or whether the nerve cells of the epileptic brain are abnormally sensitive to a change in acid-base equilibrium which is within normal limits. The present series of experiments were performed in an attempt to answer the first alternative. They comprise a study of the adjustment of the acid-base balance during and