1. Active and acid-activable inactive renin were measured in renal venous and arterial plasma of 18 patients with essential hypertension (EHT) and 19 patients with renovascular hypertension (RVHT). In seven patients with EHT and in 11 patients with RVHT measurements were made before and 25–35 min after an intravenous injection of 300 mg of diazoxide.

2. Under basal conditions the renal vein to artery ratios for active and inactive renin in EHT ranged from 0·71 to 1·96 and from 0·68 to 1·44 respectively. In 14 patients with RVHT the renal vein to artery ratio for active renin on the affected side was above the range found in EHT and in six of them the renal vein to artery ratio for inactive renin was also elevated.

3. The diazoxide-induced release of active renin from kidneys, which had a stenotic artery but were not seriously contracted, was associated with a fall of the renal vein to artery ratio for inactive renin to a value below 1·00.

4. The results indicate that changes in the release of active and inactive renin do not always run in parallel. The findings are compatible with the hypothesis that circulating inactive renin can be activated in the kidney.

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