THE total potassium content of the human body1 is of considerable interest from the point of view of physiology and in connexion with the clinical study of certain muscular disorders. Recently it has acquired additional interest because the radioactivity of naturally occurring potassium-40 in the body constitutes the principal ‘background’ in some 4π-counters used for the measurement of the gamma contamination of human subjects2–5. While it has been known for some time that the total potassium varies considerably around a value of 150 gm. in the average adult male1,6 and while it has been suspected that fat/muscle ratio is the principal determining factor, the correlation has not been clearly demonstrated.