- Medication withdrawal group: Patients come to NIH for 2 successive days at the beginning of the study for blood and urine tests (including one 24-hour urine collection) and to receive iothalamate (a chemical used to measure kidney function). Iothalamate is delivered over 24 hours through a needle placed in the abdomen (or elsewhere) via a pump similar to pumps that some diabetics use to deliver insulin. Patients then stop taking their ACE inhibitor or ARB medication. They monitor their blood pressure every day and return to NIH after 1, 2 and 4 weeks for blood tests. During week 4, the iothalamate infusion is repeated, and blood and urine samples are collected as at the beginning of the study. Patients then resume taking their ACE inhibitor or ARB once a day with the dose being increased at 2-week intervals. They come to NIH weekly after 1 week and then every other week for blood tests. Four weeks after reaching the highest FDA-recommended dose of medication tolerated, the iothalamate infusion and blood and urine collections are repeated.