THE INTRACELLULAR CONTENT OF POTASSIUM is one of the determinants of some of the physiologic actions of this important ion. In clinical practice the estimation of cellular potassium is not practical. It is well known that serum potassium (KS), despite clinical usage, often does not reflect the intracellular or total body content (KE) of this cation.1It has been implied that red blood cell potassium concentration (KRBC) is superior to serum potassium as an indicator of changes in cellular potassium.2,3 Since the determination of KRBC is simple, it was deemed important to extend these studies in conditions expected to produce changes in potassium balance. Method KRBCand KSwere measured simultaneously for the following subjects: (a) 36 normal, including some on anticoagulant therapy; (b) 29 chronically maintained on diuretics, including a cirrhotic patient with ascites; (c) 18 who were in different stages of