1. The acute effects of the kallikrein inhibitor aprotinin (498 ki.u./min), and the kininase II inhibitor SQ 14 225 (250 μg), given intravenously during saralasin-induced angiotensin blockade, were studied in conscious sham-operated rats and rats with benign and malignant two-kidney, one-clip Goldblatt hypertension during dietary sodium restriction.

2. The blood pressure of conscious sham-operated rats increased significantly in response to aprotinin. It remained unchanged after SQ 14 225 in contrast to the significant vasodepressor effect seen when SQ 14 225 was given to the same rats under surgical stress and pentobarbital anaesthesia.

3. Benignly hypertensive rats showed a consistent vasopressor response to aprotinin and a marked vasodepressor response to SQ 14 225. The effects of both inhibitors were markedly and significantly blunted in malignantly hypertensive rats.

4. Our demonstration that two agents with known opposite actions on the kallikrein—kinin system produced predictable and opposite effects on blood pressure may indicate that this system is involved in the homeostatic regulation of blood pressure. It may play an important antihypertensive role in benign two-kidney, one-clip Goldblatt hypertension, a role which might be impaired in malignant hypertension.

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