Plasma renin concentration was measured in 19 patients undergoing maintenance haemodialysis for chronic renal failure. The highest plasma renin concentrations were found in the two patients in whom bilateral nephrectomy was necessary to control blood pressure. Normal or moderately raised renin values were found in the remaining patients in whom blood pressure was more readily controlled. Bilateral nephrectomy reduced plasma renin concentration in each of 6 patients, although renin-like activity did not disappear from blood in the months following the operation. Marked loss of weight during dialysis increased plasma renin concentration whereas lesser reduction in weight usually produced a fall in plasma renin concentration.