Treatment with vasodilating drugs specifically reverses the major hemodynamic abnormality of primary hypertension. Hypotensive effectiveness of vasodilators is limited by three responses of hypertensives to vasodilation-induced blood pressure reduction, including reflex increases in sympathetic activity that raise cardiac output, augmentation of plasma renin activity, and sodium retention with plasma volume expansion. Concomitant administration of propranolol suppresses the first and second responses, and diuretics prevent the third. In treating chronic hypertension, vasodilators should be coadministered with both drugs. Potentiated in this fashion, well-tolerated doses of hydralazine (200 mg/day) can normalize the hemodynamics of most hypertensives. Guancydine and, particularly, minoxidil are promising antihypertensive vasodilators but remain in the investigative stages. Nitroprusside and diazoxide are powerful intravenous vasodilators and are effective for hypertensive emergencies. They differ in time course of action and in effect on cardiac output.