The response of plasma aldosterone to sequential ultrafiltration dialysis has been studied during 16 sessions and compared with the response to 12 conventional sessions of hemodialysis. During the isolated ultrafiltration phase an important and significant increase in plasma aldosterone and plasma renin activity was observed. There was a substantial removal of extracellular fluid, but no change in plasma biochemistry. During conventional hemodialysis over the same time scale the concentration of plasma aldosterone decreased significantly, plasma renin underwent a small increase and plasma osmolality and plasma potassium decreased. Several possible pathophysiological mechanisms are discussed. We conclude that rapid isosmotic ultrafiltration without change in plasma potassium or sodium, the maintenance of plasma potassium at normal elevated values, and the increase of angiotensin II are factors favoring aldosterone biosynthesis.