The saralasin test was performed in 68 hypertensives. A clear-cut dependence of the test results on initial plasma-renin concentration and particular sodium balance was demonstrated. Because of this dependence the saralasin test should be performed only under constant conditions. A mild stimulation of the renin-angiotension system by salt restriction to a mean sodium excretion of 50 mmol daily and 80 mg furosemide by mouth 12 hours before the test seems best. In this way essential and renovascular hypertension could be distinguished with considerable reliability (P less than 0.001). Among patients with essential hypertension one could clearly separate those with high plasma-renin concentration from those with a normal or low one. Among patients with renovascular hypertension those with haemodynamically significant renal artery stenosis could with high probability be distinguished from those with non-effective stenosis. A positive saralasin test without testing the function of the normal contralateral kidney does not provide an indication for operation.