1 In a placebo-controlled study, the respective anti-hypertensive effects of hydrochlorothiazide and hydrochlorothiazide plus the beta-adrenoceptor blocker acebutolol were assessed in 18 patients with moderately severe essential hypertension. 2 Hydrochlorothiazide 100 mg daily decreased the mean supine blood pressures from 163/107 mmHg to 150/103 mmHg. Addition of acebutolol in a single-blind fashion in doses up to 800 mg daily reduced mean supine pressure to 137/95 mmHg. Further increases in acebutolol dosage to a maximum of 2000 mg daily in 13 patients whose hypertension was not well controlled on lower doses resulted in a mean supine blood pressure of 132/92 mmHg.