The purpose of the study was to determine the early haemodynamic effects of injectable acebutolol and to study in parallel the changes in plasma renin activity (P.R.A.) in 11 patients, most of whom were affected with labile arterial hypertension. The haemodynamic measurements and P.R.A. determinations were carried out before and 5 and 15 mn after very slow intravenous injection of 10 mg acebutolol. The results obtained under these conditions were as follows: 1) Decrease in cardiac index (I.C.). It was particularly significant after 5 mn, since the I.C. decreased from 4.43 to 3.75 1/mm (p less than .01). It was essentially due to a decrease in heart rate, that fell from 90.45 to 77 beats/mn (p less than .001), while stroke volume changed virtually not. 2) Decrease in blood pressure, more marked on systolic blood pressure (110 mm Hg after 15 mn as against 154 mm Hg before injection; p less than .001), associated with a decrease in left ventricular work (4.7 kgm/mn/m2 after 5 mn as against 6.2 kgm/mn/m2 before injection; p less than .001), without significant changes in total systemic arterial resistances. 3) Increase in diastolic pulmonary arterial pressure (10.4 mm Hg after 5 mn against 8.09 mm Hg before injection; p less than .001), testifying to a slight left ventricular myocardial depression. 4) Decrease in supine P.R.A. level (0.72 nanogram/1/mn after 5 mn as against 1.15 nanogram/1/min before injection; p less than .01). A significant correlation was found between this decrease in P.R.A. and that in I.C., testifying to a close parallelism between the inhibition of cardiac beta-1 receptors and that of the receptors involved in renine secretion. 5) The tolerance of injectable acebutolol appeared to us to be excellent.