Total body sodium and chloride were measured by neutron activation analysis, and total body potassium was measured by whole body counting in ten male patients with endstage renal disease requiring chronic maintenance hemodialysis. The extracellular fluid volume was estimated from the simultaneously measured volume of distribution of bromine 77 and sodium 24. Total body water was estimated from the volume of distribution of triated water. Total body sodium and chloride were significantly increased above normal measured values, but total body potassium was not significantly different from normal. The increase in total body sodium could be attributed to an increase in exchangeable sodium because no significant change in the nonexchangeable portion of total body sodium could be detected. Extracellular fluid volume, estimated fro three different techniques (total body chloride, sodium 24, and bromine 77), and total body water were increased significantly above normal values. Extracellular fluid volume was correlated with exchangeable sodium, and total body water was closely correlated with total cation content (exchangeable sodium and total body potassium). The calculated intracellular fluid volume was decreased and was closely correlated with total body potassium. Despite adequate control of uremia by chronic maintenance hemodijalysis, body electrolyte composition and the distribution of body water remain significantly different from normal.