1. The rat renal papilla was selectively destroyed by 2-bromoethylamine hydrobromide; increasing doses produced a graded severity of histological damage, polyuria and a reduction in urinary prostaglandin E2 excretion.

2. Destruction of at least half of the papilla caused significant hypertension, but plasma renin concentration and plasma creatinine did not change.

3. After graded medullary damage Goldblatt two-kidney, one-clip hypertension was induced. Four weeks later the severity of hypertension was similar in medulla-damaged and medulla-intact groups.

4. When Goldblatt two-kidney, one-clip hypertension was reversed by removal of the renal artery clip, the blood pressure fall was less when the inner renal papilla was ablated than when it was intact, although plasma renin concentration and sodium balance were similar. The final blood pressure was similar to that observed in normal rats treated with 2-bromoethylamine compared with their respective controls.

5. These findings are consistent with the hypothesis that the inner renal medulla produces a vasodepressor that influences blood pressure in normal rats, and also lowers blood pressure when two-kidney, one-clip hypertension is reversed surgically.

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