In the course of work already reported (1) (2) on the relationship of water balance to the occurrence of convulsions in severely epileptic children, it was found that seizures, after having been brought under control by the production of a deficit in the body water, could be made to recur practically at will by the rapid reestablishment of a positive water balance with pituitary antidiuresis. The investigation reported in the present paper was undertaken with the purpose of further elucidating the mechanism of this reaction, which appeared to us to open a new avenue of approach to the complex problem of etiology in epilepsy.In order to avoid misinterpretations based upon the spontaneous occurrence of seizures, patients were selected who were known to have convulsions infrequently but with a fair degree of regularity. It was believed that any fundamental difference that might be found between the response of the normal subject to an artificially induced disturbance in the body fluid relationships and that of the epileptic patient during his usual period of freedom from seizures, might furnish valuable information regarding the underlying physicochemical pathology in epilepsy. The possibility of utilizing the observed reaction of the severe epileptic to an enforced positive water balance as a diagnostic procedure in mild or obscure cases also occurred to us early in the study.