Methods We will select study participants from adult hemodialysis patients treated thrice weekly at Shaare Zedek Medical Center Dialysis Unit. Men and women will qualify for the study if they were on hemodialysis therapy for more than 3 months, have an average predialysis plasma potassium concentration less than 5.6 mEq/L at the time of enrollment and have nil or minimal urine output (<500 mL/24 h). All participating women of childbearing age will have a negative pregnancy test result before entering into the study. Additional exclusion criteria will include a known allergy to Spironolactone or Eplerenone; any acute illness; hypotension, defined as a predialysis systolic blood pressure less than 100 mm Hg; severe hypertension (predialysis systolic blood pressure >180 mm Hg and/or diastolic blood pressure >100 mm Hg); decompensated heart failure; inability to give informed consent; and noncompliance. The Institutional Review Board of Shaare Zedek Medical Center will be asked to give approval to the study protocol. All participants will give written-informed consent.