Previously reported results for exchangeable potassium in advanced renal failure have been conflicting. We have studied, therefore, 13 patients in advanced chronic renal failure with a mean urea clearance of 3 ml/min and a mean creatinine clearance of 4.1 ml/min, and without evidence of unusual extrarenal potassium losses. The results of exchangeable potassium in both males and females were not significantly different from normal (43.3 mEq/kg in males and 35.9 mEq/ kg in females). In the majority of patients overall potassium secretion was observed. Our results do not support arbitrary potassium supplementation in patients treated conservatively for renal failure in an effort to improve their glucose tolerance.