1. We have measured plasma concentrations of renin, renin substrate and angiotensins I and II as well as plasma renin activity in nine patients with severe or malignant hypertension during treatment with captopril, hydrochlorothiazide and propranolol.

2. On captopril and hydrochlorothiazide the plasma concentrations of renin substrate and angiotensin II decreased markedly, while renin and angiotensin I levels were increased.

3. The changes in renin substrate concentration suggest a consumption of substrate induced by an increased renin release. Further, the positive feedback of angiotensin II on hepatic renin substrate synthesis may be inhibited.

4. The sequential changes in renin release during captopril treatment should be monitored by measuring plasma renin concentration since plasma renin activity measurements will be profoundly influenced by the marked changes in plasma renin substrate concentration.

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