To a large degree the therapy of acute renal failure depends on an understanding of biochemical disturbances that result from the temporary absence of renal excretory and homeostatic functions. Important among these disturbances are changes in the volume and composition of body fluids, particularly with respect to electrolyte constituents. While certain manifestations of these changes are frequently recognized on the clinical level, relatively little quantitative data are available from which can be derived a general pattern of body fluid alterations in this disease. One reason for this paucity of information, aside from the limitations of investigative techniques, is the inherent difficulty of performing adequate studies on critically ill patients.Sirota and Kroop (1) observed in 4 oliguric patients an expansion of inulin space roughly commensurate with estimated positive fluid balance. They postulated that the oliguric phase of acute renal failure is associated with cellular dehydration resulting from a shift of water out of cells, and that hyponatremia may result in part from extracellular dilution and in part from a shift of sodium into cells. The observation by Schwartz, Tomsovic, and Schwartz (2) of greater expansion of inulin space than of D20 space in one anuric child would conform with this concept. Iseri, Batchelor, Boyle, and Myers (3) also suggested that the hyponatremia and hypochloremia of oliguria may be due in part to cellular uptake of sodium and shift of water from cells to extracellular fluid. The role of catabolism in augmenting total body water through water of oxidation and release of preformed water was evaluated by Hamburger and Richet (4), largely by inference from observations made during post-oliguric diuresis. Changes in the volume and distribution of body water were also studied in anuric dogs by Hamburger and Mathe (5), who found close agreement between the expansion of total body water as measured by D20 spaces and the amount of water derived from metabolic processes. On the basis of extensive clinical observation and a limited number of isotopic dilution studies, Merrill (6) has enumerated some of the fluid and electrolyte changes frequently seen in prolonged acute renal failure, as follows: 1) an increase in total body water, 2) an increase in total body sodium with a decreasing serum concentration, 3) a decrease in total body potassium with an elevated serum level, and 4) an increased extracellular fluid volume.The primary purpose of the present study was to obtain more complete information