The effects of a single 25 mg oral dose of captopril on blood pressure, heart rate and circulating renin, angiotensin I, angiotensin II, bradykinin and catecholamine levels were examined in untreated patients with essential (n = 10, Group I), accelerated (n = 6, Group II) and renal hypertension (n = 8, Group III) studied on a normal sodium diet. 2. Mean blood pressure fell only slightly in Group I patients, (113 +/- 3 to 109 +/- 3 mmHg at 60 minutes) but a greater fall was observed in Group II (153 +/- 8 to 135 +/- 11 mmHg) and a marked fall in Group III, (136 +/- 3 to 114 +/- 5 mmHg). There were no significant changes in heart rate in any group. 3. Plasma angiotensin II levels were significantly reduced 30 minutes after captopril in all three groups and returned toward resting values after four hours. The falls in plasma angiotensin II levels were accompanied by reciprocal increases in blood angiotensin I and plasma renin, but blood bradykinin and plasma catecholamine concentrations remained unchanged. 4. Resting plasma renin levels showed considerable overlap in the three groups and the mean renin values were not significantly different in the three groups. After captopril a marked rise in plasma renin concentration (greater than 2.5 ng/ml/hr) was observed in seven patients in Group III, including all six patients with renovascular disease. In contrast, none of the patients with essential hypertension and only one patient with accelerated hypertension had such an increase. Determination of the acute renin and blood pressure responses to converting enzyme inhibition with a single oral dose of captopril appears to be useful in identifying patients with renovascular hypertension.
