The availability of artificial isotopes of physiologically important elements has made possible measurement of the amount of these constituents in the living human body by application of the principle of isotope dilution. A known amount of isotope is administered and allowed to exchange with its naturally occurring brothers while its excretion is measured. When equilibrium is attained, the quantity of element in which the isotope is "diluted" is inversely proportional to the concentration of artificial isotope in natural isotopes (specific activity) of any body sample. This quantity of element we refer to as the body content of "exchangeable" element, which in the case of complete exchange is the total amount of element in the body. Development tracer was carried out (2), and, independently, normal values of the exchangeable sodium in man were determined with Na24 in another laboratory (3).The primary purposes of this study were to develop a method for the measurement of the amount of exchangeable potassium in the living human and to determine such values in healthy adult men. It became evident early that complete exchange was not attained during the period of observation afforded by safe tracer doses of K42 and that choice of a representative sample for analysis was a major problem. It was also evident that the significance of "normal" measurements would be enhanced by examination of the correlations that might exist with other body indices and that might permit extension of our data to studies of other healthy persons and pathologic states.The body content of potassium is an anthropometric value comparable to body weight, surface area or height and correlations with such measurements were investigated for possible clinical applications. Potassium on a weight basis is an extremely small fraction of the body, approximately 0.2 %, which assumes importance because it is the predominant cation of intracellular fluid. We should expect, then, to derive the greatest significance from studies of potassium in relation to intracellular water but present methods for measuring this phase of body water are indirect and difficult. Despite reservations cell water was estimated in this study as the difference between total body water and thiocyanate space. The almost exclusively intracellular position of potassium renders it probable that the body content of potassium is a good index of cellular mass. This possibility was explored by studying the correlations with total body water, basal oxygen con-1280

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