20 patients with severe essential hypertension (average blood-pressure 211/123 mm Hg) had an inadequate fall in blood-pressure with beta blockade alone. They were given in random order either 5 and then 10 mg of bendrofluazide a day or prazosin 2 mg three times daily rising to 5 mg if required. The trial was a within-patient comparison of the two drug regimens. 10 patients who did not achieve a satisfactory fall in pressure with either agent were then given all three drugs together. When bendrofluazide 5 or 10 mg was added to beta blockade there was an average fall in mean blood-pressure, standing, of 13%. When prazosin was added to beta blockade the average fall in mean blood-pressure, standing, was 16%. 18 patients who completed the trial had an average final blood-pressure, standing, of 139/93 mm Hg. In the prazosin period 8 patients continued to complain of dizziness after the first 24 h. With bendrofluazide serum-potassium levels fell below 3-6 mmol/l in half the patients within the first two weeks of treatment. It is concluded that patients with essential hypertension already treated with beta blockade who need an additional agent will get a further fall in blood-pressure with 5 mg of bendrofluazide. Prazosin appears to be a potent and appropriate third agent.