Subjects will be prepared and will be studied in approximate metabolic balance at 150 meq Na+/d. After approximate metabolic balance is achieved (estimated 5 days), subjects will receive either fenoldopam infusion or vehicle infusion on Day 6 and the opposite agent (fenoldopam or vehicle) will be infused on Day 7. The order of infusion will be randomized and the study will be conducted in double-blind fashion. Vehicle will be D5W and will be infused during the control period using the same rate as the post control period. At 1100 h on study Day 6, an i.v. infusion of fenoldopam or vehicle will be initiated and continued for 3h. The fenoldopam infusion rate will be 0.05 μg/kg/min. This infusion rate of fenoldopam has resulted in a greater than 2-fold increase in UNaV without alteration of systemic hemodynamic function in normal subjects (30). Plasma fenoldopam levels will be monitored to document delivery of this agent into the circulation. BP and heart rate will be measured in duplicate every 10 min during the experimental period (1100-1400h). The experimental period will be followed by a 2-hour post-control period (1400h-1600h) during which vehicle will be infused. Blood and urine samples will be obtained every 30 min and will be analyzed for Na+, K+, lithium and creatinine concentrations and osmolality. In addition, blood samples will be analyzed for PRA, cyclic AMP (blood and urine), plasma ANG II and aldosterone concentrations at 1045, 1215, 1345 and 1545 h. After completion of a post-control period at 1600h, the subjects will be allowed to be out of bed but remain as inpatients and receive the same diet as previously. No food is given after 2400h on Day 6. On Day 7, the identical protocol as on Day 6 will be repeated except that the opposite agent (fenoldopam or vehicle) will be infused. The subjects will be discharged from the GCRC on their previous diet.