SummaryTotal body water, exchangeable sodium and exchangeable potassium were repeatedly measured with radioisotope dilution techniques in six patients in the end-stage of chronic renal failure, and maintained alive with repeated dialyses (haemodialysis and peritonea] dialysis). This study attempted to correlate blood pressure with exchangeable sodium in such patients. In lour subjects, which were ultimately submitted to renal transplantation, the effect of bilateral nephrectomy was also examined.In a first group of three patients, exclusively treated by dialysis, hypertension was not severe, and retinal changes were minimal, or absent, or disappeared during chronic dialysis. In these three patients, exchangeable sodium was high, and in two of them a close relationship between blood pressure readings and sodium poof values was observed.In the second group (four patients), hypertension was of the malignant type, with marked retinal changes, heart failure, and/or cerebral dysfunction. In three patients, exchangeable sodium was normal or slightly elevated; it was not measured in the fourth subject. In the four patients, bilateral nephrectomy was followed by dramatic and sustained lowering of the blood pressure. In two of these patients, the normotensive state was not accompanied by any reduction of the sodium pool. In the other patient, the collected data were insufficient to dissociate the effect of nephrectomy from that of a fall of exchangeable sodium.These observations suggest that at least two factors contribute to raise the blood pressure in the terminal stage of Bright's disease : an excessively large sodium pool and some unknown pressor renal activity (renin ?). The respective influence of each of these two factors on blood pressure might vary considerably from one patient to another.