The aim of this study was to compare the haemodynamic effects ofacebutolol, a ~I-selective adrenoceptor blocker with intrinsic sympathomimetic activity, and propranolol, a nonselective ~-adrenoceptor blocker without intrinsic sympathomimetic activity. 44 males aged 35 to 59 (mean 49.7 ± 1.0) years with mild to moderate arterial hypertension (diastolic blood pressure 95 to 114mm Hg) without severe concomitant diseases requiring medication, and without contraindications to ~-adrenoceptor blockade, were included in this double-blind randomised parallel group study. It was shown that during 12 weeks' treatment the magnitude and duration of the antihypertensive effect of acebutolol 400 or 800mg daily did not differ from that of propranolol 80 or l60mg daily. However. there was a trend towards more patients achieving a full or partial antihypertensive effect with acebutolol than propranoloL Moreover, bradycardia was more pronounced with propranoloL The antihypertensive effect of acebutolol was observed after 2 weeks and maintained throughout all 12 weeks of treatment. Unlike propranolol, acebutolol did not cause such unwanted haemodynamic effects as decreased cardiac output and increased total peripheral resistance. Acebutolol was well tolerated. NSNS NS NS NS NS NS NS NS NS NS NS NS NS NS NS