The aim of this study was in vivo testing of the action of three newly synthesized potential ultrashort acting beta-blockers on the heart rate in the laboratory rat. The tested substance 44Bu was administered to animals with induced tachycardia, in the form of an intravenous bolus in general anaesthesia.Doses at concentrations of 1.5 mg⋅kg -1 , 2.5 mg⋅kg -1 , and 3.5 mg⋅kg -1 of body mass were tested and the efficacy was compared with placebo. For the heart rate monitoring a computer electrocardiograph was used. Significant (p < 0.05) heart rate decrease was recorded for all three tested doses, minimally up to the 14 th minute following the intravenous administration.Bradycardic effect of the compound 44Bu was compared with the action of esmolol under the same experimental conditions. The effects of the compound 44Bu and esmolol were not different in the onset, but in the depth of the heart rate decrease, above all at higher concentrations. It was experimentally verified, that the compound 44Bu has the properties of an ultrashort acting beta adrenergic receptor blocker.Pharmacology, ester-functional group, bradycardic effect, heart rate, rats