The renin-aldosterone system and plasma insulin were studied in 19 patients with familial Mediterranean fever (FMF). Their relationships to serum potassium level at rest and before and after oral glucose loading are described. An interesting finding is the occurrence of hyperkalemia in the absence of oliguria, in the advanced stages of renal failure. No differences were found in the activity of the renin-angiotensin-aldosterone system to explain these variations in serum potassium found in some of the patients. The response of the renin-aldosterone system to glucose loading showed no abnormality, and the regular relationship between serum potassium, plasma renin activity (PRA), aldosterone, insulin, and plasma pH is maintained. Levels of insulin, potassium, and bicarbonate in serum or plasma pH were found similar in FMF patients with normal renal function with and without proteinuria. Further decrease in renal function due to the progression of the underlying disease is manifested by an increase in FENa+, and FE K+, and a hyperchlo-remic metabolic acidosis, as is the case in other patients with chronic renal failure.