1. Three renin-stimulating methods for detection of low-renin hypertension have been compared. First, renin activity was measured in hospital patients after 5 days of sodium restriction and 3 h ambulation. Secondly, renin activity was measured after frusemide stimulation [0.42 mmol (140 mg) in 18 h] and 3 h ambulation. Thirdly, renin activity was measured after 5 days of chlorthalidone treatment [0.3 mmol (100 mg)/day] and 3 h ambulation. The last two tests were done with the subjects as out-patients without any dietary regimen. 2. In eleven normotensive control subjects and twenty hypertensive patients the results after frusemide were not comparable with those after sodium restriction since the frusemide test did not identify the same renin-suppressed hypertensive subjects as the sodium-restriction procedure. 3. After 5 days of chlorthalidone treatment the renin values in eleven control subjects as well as in thirty-eight hypertensive patients were significantly higher than after sodium restriction. The values obtained after each procedure were closely correlated. 4. Thus the out-patient chlorthalidone procedure identified similar sub-groups of patients as having low- or normal-renin hypertension as did the in-patient sodium-restriction test.