1. Minoxidil is a direct-acting vasodilator which has been used in combination with propranolol and hydrochlorothiazide to lower blood pressure in hypertensive patients. 2. The therapeutic efficacy of minoxidil was compared with that of hydrallazine used in the same combination in eleven patients refractory to standard antihypertensive therapy. 3. The recumbent blood pressure of these patients on propranolol and hydrochlorothiazide alone averaged 191/128 mmHg and decreased to 142/92 mmHg during addition of minoxidil and only to 168/108 mmHg during hydrallazine treatment. 4. In addition to its greater efficacy, minoxidil exhibited a prolonged duration of action and there was no evidence of diminished response during treatment for as long as 2 years. 5. The paucity of side effects and the powerful activity of minoxidil in combination with the other drugs suggests that this agent may be of great value in the chronic therapy of hypertension.