1. Water and electrolyte status in chronic renal failure has been measured by skeletal muscle biopsy. These results have been compared with a series of muscle biopsy specimens taken from the same muscle (vastus lateralis) of normal subjects. 2. The water content, especially of the extracellular phase, is increased in chronic renal failure: the extracellular sodium and chloride contents are also increased. These changes tend to be more marked as the severity of renal failure increases. 3. The intracellular water and potassium contents in chronic renal failure are little different from those in normal patients. 4. A significant correlation between diastolic blood pressure and extracellular water content is shown and this is especially marked in more severe degrees of renal impairment. 5. These results support the belief that hypertension in chronic renal failure is salt and water dependent, and that hypertension in these patients can be controlled by removal of salt and water.