Eighteen patients whose clinic blood pressure (BP) remained over 95 mmHg despite treatment with captopril 50 mg twice daily plus frusemide 40 mg twice daily were randomised in a crossover study to four weeks' treatment with once daily atenolol 100 mg, slow release propranolol 160 mg or placebo. The reduction in BP on atenolol was superior to that on both propranolol and placebo. The mean supine BP 24 hours post dosing were 177/110 mmHg (placebo), 173/109 mmHg (propranolol) and 164/100 mmHg (atenolol). The corresponding mean heart rates were 77 bpm (placebo), 63 bpm (propranolol) and 62 bpm (propranolol) and 62 bpm (atenolol). The difference in hypotensive efficacy between atenolol and propranolol is not readily explained but our study shows that atenolol has a clinically useful supplementary effect on BP. Refractory hypertension remains an important clinical problem and further studies are required to establish the optimum combination of drugs that should be used with captopril in order to achieve 'target' BP in patients with moderate to severe hypertension.