Minoxidil, a new peripheral vasodilator, given orally to hypertensive men in single doses of 5-25 mg, produced no haemodynamic changes within one hour after administration. After repeated oral doses within 24 hours to a total of 15-45 mg and after 10 mg t.i.d. orally for one week, moderate decreases in BP were seen concomitant with tendencies to increased heart rate and cardiac output. Clinically, oral minoxidil 10-50 mg daily in combination with diuretics and adrenergic beta-receptor blocking agents achieved an improved BP control in five patients with sustained arterial hypertension and unsatisfactory response to previous treatment. However, in four of the five patients minoxidil had to be withdrawn because of side-effects. It is concluded that minoxidil, producing a hypotensive effect of slow onset, may find a place as a therapeutic addition to symptomatic patients with severe and therapy-resistant hypertensive cardiovascular disease, provided adequate measures are taken to counteract side-effects, especially water retention and development of oedemas.