A standardized test for renin responsiveness, employing the dual stimulus of upright posture and the loop diuretic furosemide, was applied to 19 hypertensive patients in the untreated state and during therapy with the antihypertensive agents guanethidine and methyldopa. During therapy with guanethidine, 6 of 10 patients with "low-renin essential hypertension" experienced elevations of plasma renin activity to levles ordinarily diagnostic of "normal-renin" hypertension (P less than 0.05), whereas methyldopa had no significant effect on plasma renin activity in either "low-renin" or "normal-renin" patients. It is suggested that methyldopa has a negligible influence on renin responsiveness when stimulated under the above conditions and that it may be used during assessment of plasma renin activity in hypertensive patients whose blood pressure is too severely elevated for temporary withdrawal of therapy.