Abstract: A method for measuring total body cell mass, total body extracellular mass, total exchangeable potassium and total exchangeable sodium of a subject by using bioelectrical resistance and reactance. The method includes the steps of connecting electrodes attached to an electrical impedance analyzer to the subject followed by introducing a high frequency, low amperage current through the electrodes into the body of the subject. The bioelectrical resistance and reactance of the subject are then measured by the impedance analyzer. Finally, the total body cell mass, total extracellular mass, total exchangeable potassium and the total exchangeable sodium of the subject are determined by applying the measured resistance and reactance to equations derived from multiple linear regression analysis of total body resistance and reactance of a selected group of subjects.