A stable volume and composition of extracellular fluid are essential for normal functioning of the body. Since the kidney is primarily responsible for regulating extracellular fluid, loss of kidney function should have catastrophic consequences. Fortunately, even with loss of more than 90 percent of renal function, a remarkable capacity to regulate body fluid volumes and sodium and potassium persists. Nevertheless, this capacity is limited to chronic renal disease and this has important consequences for clinical management of these patients. How can sodium and potassium homeostasis be assessed? Methods for evaluating the steady-state regulation of sodium include measurement of body fluids and their distribution in different compartments and measurement of exchangeable and intracellular sodium. Short-term regulation of body sodium can be assessed from measurement of sodium balance during changes in dietary salt. Potassium is predominantly contained within cells and thus the assessment of its regulation requires special emphasis on measurement of steady-state body stores and potassium distribution across cell membranes. However, the methods used to make all of these measurements require assumptions that may not hold in the altered state of uremia. This raises problems in interpretation requiring critical analysis before conclusions can be made regarding sodium and potassium homeostasis in patients with chronic renal failure. This review focuses on abnormalities of body fluids, sodium and potassium in patients with creatinine clearances of less than 20 ml/min due to chronic renal failure and the impact of conservative therapy, dialysis and renal transplantation on these patients.