1. Thirty-four patients with essential hypertension and 37 with proven renovascular hypertension were tested with saralasin after frusemide-induced diuresis.

2. Variables used for analysis were: pre-saralasin plasma renin activity (PRA), post-saralasin PRA, and ratios of systolic and diastolic blood pressures (saralasin blood pressure/control blood pressure).

3. Stepwise linear discriminant analysis demonstrated that the most significant variables were diastolic ratio, systolic ratio, and post-saralasin PRA.

4. Minimal misclassification (14·1%) was achieved by combining diastolic ratio, baseline supine PRA and post-saralasin PRA; but using post-saralasin PRA alone resulted in misclassification of only 15·5% (7 essential hypertension and 4 renovascular hypertension).

5. Thus, PRA increased disproportionately after saralasin in renin-dependent renovascular hypertension because of the fall in blood pressure and/or interruption of the short, negative angiotensin II feedback loop.

6. It is concluded that the rise in PRA after the combined stimuli of volume depletion and saralasin is more sensitive than the fall in blood pressure in discriminating between essential hypertension and renovascular hypertension, although minimal misclassification is achieved by analysing baseline PRA, diastolic ratio and post-saralasin PRA simultaneously.

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