 Following the 180-minute timed void, all voluntary voids will be collected but the volume of oral fluid intake will match the urine output i.e. water loading to stop. The nursing staff will continue to monitor fluid balance. For all urine samples, a bladder scan will be conducted immediately before and immediately after the subject urinates to assess the completeness of bladder emptying. Following the equilibration period for inulin 90 minutes and 125I-iothalamate 60 minutes, four timed urinary clearance periods of at least 30 minutes will begin. Blood samples for urinary clearance measurements will be taken following each bladder emptying. Blood samples for plasma clearance measurements will be taken at approximately 10, 30, 120, 150, and 240 min. For patients with estimated GFR 30 mLmin per 1.73 m2, an additional blood sample will be collected at 360 min. Subjects are free to move around during the GFR test. 