In order to evaluate different methods of estimating intracellular content of potassium, determinations of potassium were made in erythrocytes, in muscle biopsy specimens and in the total body. The total body potassium (TBK) was related to body weight (Bwt), lean body mass (LBM), dry LBM, soft fat-free solids (SFFS), total body water (TBW) and calculated intracellular fluid (ICF). Sixty-two healthy subjects and 80 patients with various diseases and therapy known to influence the potassium metabolism were studied. Statistically significant correlations were found between potassium content in muscle biopsies related to fat-free solids and TBK/Bwt (r = +0.56, P less than 0.05), TBK/LBM (r = + 0.58, P less than 0.01), dry LBM (r = + 0.54, P less than 0.05) and TBK/SFFS (r = + 0.68, P less than 0.01). A correlation was found between muscle potassium related to intramuscular water and TBK/TBW (r = + 0.47, P less than 0.05). No correlations were found between EK and TBK/LBM, TBK/dry LBM or TBK/ICF. Between EK and serum potassium a negative correlation (r = -0.31, P less than 0.05) was found in a group of untreated hypertensives. It is concluded that the quotients of TBK (TBK/LBM, TBK/TBW and TBK/SFFS) might be used in the study of intracellular potassium--at least in the study of patient groups.