1. Chemical renal medullectomy was produced in rats by injection of 2-bromoethylamine hydrobromide. Plasma creatinine and blood pressure were unchanged although urine volume was increased fourfold.
2. Left renal artery constriction resulted in similar degrees of hypertension in both intact and medullectomized rats. This was associated with a significantly smaller rise in plasma renin concentration in the latter.
3. Blood pressure in conscious intact hypertensive rats became normal within 24 h of unclipping whereas blood pressure of medullectomized rats remained significantly elevated.
4. The presence of an intact renal medulla is essential to the complete reversal of two-kidney, one-clip hypertension in the rat. This may reflect the loss of a medullary vasodepressor system.