1. The effect of aprotinin-induced blockade of the kallikrein-kinin system on haemodynamic and biochemical responses to converting enzyme inhibition by SQ 14 225 was evaluated in 26 patients with essential hypertension.

2. SQ 14 225 lowered blood pressure in high, normal and low renin hypertension. In low and normal renin patients, but not in high renin patients, the acute blood pressure-lowering effect of SQ 14 225 could be overcome by aprotinin. Aprotinin infusion produced small vasopressor effects in all groups of patients.

3. Aprotinin lowered the level of circulating active renin but not that of inactive renin.

4. It is concluded that in low and normal, but not in high, renin hypertensive patients activation of the kallikrein-kinin system is responsible for the acute blood pressure reduction observed with converting enzyme inhibition.

5. With long-term converting enzyme inhibition kallikrein-kinin system activation seems to play only a minor role.

6. The kallikrein-kinin system may be involved in the regulation of blood pressure.

7. There is no direct evidence of a participation of kallikrein in the activation of prorenin in vivo.

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