The renin-angiotensin-aldosterone system and the acidification capacity of the renal tubule were studied in 13 diabetic patients with chronic renal failure. As a whole, the group showed hyporeninaemic hypoaldosteronism (HH). Studied alone, 12 of the 13 patients presented the requirements for HH. This group showed hypercholaemic hyperkalaemic metabolic acidosis with a disturbance in renal acidification which may be classified as Type IV renal tubular acidosis. The results of this group were compared to those of another two groups; one of diabetic patients without chronic renal failure and the other with chronic renal failure (C Cr less than 40ml/min); both were seen to show different behaviour to that of the group affected by the two processes.