Total body sodium has been measured in three normal adult males by whole-body neutron activation analysis. The values obtained are considerably lower than previously accepted values based on chemical analysis of cadavers. The presence of a significant sodium pool, not detected by isotope dilution techniques, was confirmed. The techniques described in this paper involving the neutron irradiation of living subjects have been approved by an ethical committee of Birmingham University. Clinicians and physicists were represented on the committee, which included the Radiation Protection Officers of the University and of the United Birmingham Hospitals. The work involving cadavers was done with the permission of the Inspector of Anatomy at the Home Office. © 1968, British Medical Journal Publishing Group. All rights reserved.