The overarching goal of this proposal is to definitively establish if acute water free Na storage and release occurs, if the amount stored/released is of a quantity that is physiologically relevant, and if the location is intracellular vs. extracellular.

This study is designed as a an ultra-rigorous inpatient balance study. The main purpose of the study is to determine if significant mobilizable non- extracellular volume (ECV) Na storage occurs in humans. This is a randomized, blinded crossover study. Patients will be randomized to Na free 5% dextrose solution or 1.5% standard peritoneal dialysis (PD) fluid (Dianeal 1.5% low calcium PD solution with standard 132 mmol/L Na and osmolarity of 344 mOsmol/L) and will receive treatment for 5 days, following a 6 week wash out patient will return to undergo 5 days of alternate therapy.

Inclusion Criteria:

• ESRD Patients actively undergoing PD with a reliably functioning PD catheter
• Systolic blood pressure > 130 mmHg
• Serum sodium >135 mmol/L
• daily urine output < 400ml
• stable PD prescription for at least 2 months without requirement of 4.25% glucose PD solution
• PD vintage of at least 6 months
• Euvolemic as defined by the patient at their dry weight and free of any signs or symptoms of volume overload, per referring nephrologist

Exclusion Criteria:

• Poorly controlled diabetes with hemoglobin A1C>9%
• 1 or more episodes of peritonitis in the previous 6 months or active infection of the peritoneal dialysis catheter
• Anemia with hemoglobin <8g/dL
• Inability to give written informed consent or follow study protocol.
• Urinary incontinence
• Body weight < 60kg
• gastrointestinal disease that causes diarrhea or expectation of sodium losses in stool.
• use of amiodarone in the last 6 months (which will interfere with tissue iodine ascertainment).