Left ventricular function was studied in 15 patients both before and after intravenous administration of Practolol (an agent blocking selectively the heart beta-adrenergic receptors (beta 1)), while the heart rate was constant by right atrial pacing. The negative inotropic effect linked to the negative chronotropic effect of the beta-adrenergic blockage, represented approximately one quarter of the overall depression of the contractility. Right atrial pacing determined a decrease of the left ventricular end-diastolic pressure and an increase of the pacing rate by 20% above the spontaneous heart rate, after administration of 30 mg of Practolol, brought the contractility back to its control level.