Prazosin was compared with hydralazine in 39 patients (mean age 49 years), who were already on beta-blockade for arterial hypertension. They all had a resting DBP greater than 100 mm Hg on propranolol 80 mg b i d. They were randomly allocated to prazosin or hydralazine treatment. Blood pressure was measured after 4, 8 and 12 weeks. We noted a significant fall in blood pressure in both groups, in most cases to normotensive levels. Side-effects and other variables of interest are discussed. Together with beta blockade, i e propranolol, both prazosin and hydralazine give good blood-pressure control and no important differences could be found.