The fact that seizures can usually be prevented in epileptic children by fasting1or by diets relatively low in carbohydrate and high in fat2has interested us during the past three years, largely as a starting point for a study of the etiology of this disorder. We have investigated various phases of the problem with the assumption that any knowledge of the physiologic alterations produced by favorable therapeutic regimens might lead to a better understanding of the basic disturbance present, just as information regarding the effects of the seizure itself might give a clue to its mechanism. The present report is concerned with changes in the water balance of the body, a phase of the subject which is intimately related to other angles covered in previous communications from this clinic. The possible importance of this aspect of the problem was first suggested to us by the fact that