The relationship between plasma potassium concentration and the renin-angiotensin-aldosterone system was evaluated in ten patients with chronic renal failure (creatinine clearance 10–56 ml/min). Under basal conditions and following various stimulation maneuvers, normokalemic patients demonstrated normal plasma renin and aldosterone levels. Five of six patients with hyperkalemia had diminished function of the renin-angiotensin-aldosterone system; their ability to conserve sodium during salt depletion was less than that of normokalemic patients. The data suggest that the maintenance of plasma potassium levels in these patients is dependent on the presence of a normally functioning renin-angiotensin-aldosterone system; aldosterone activity may be an important determinant of sodium conservation in patients with renal failur