THE demonstration by Hodgkin and Katz1 that the process of polarization and depolarization of the cell membrane depends upon the gradients of sodium (Na) and potassium (K) across the cell membrane has become a cornerstone to our understanding of the propagation of action potentials. The possibility that abnormalities in Na or K metabolism might account for altered neuronal excitability and might be manifest clinically as disturbed behavior has intrigued clinicians and investigators. In early studies, reviewed by Altschule,2 concentrations of electrolytes in body fluids of patients with affective disorders usually were found to be normal. Changes in electrolyte balance during the course of a psychiatric illness have also been studied; cyclic mood disorders, such as manic-depressive disease, are especially suitable for this research approach. A number of early reports indicated that Na retention occurred during depression and that Na