Abstract: Carnitine (VBt) (β-hydroxy-γ-trimethylaminobutyric acid) is a naturally occurring amino acid found in all living tissues.In the present study, VBt exhibited positive inotropic and negative chronotropic effects on the experimental failing heart in situ in anesthetized cats as well as in isolated guinea pig hearts. After intravenous infusion of VBt at a constant speed of 50 mg/kg/min for 5 minutes, the MAP, LVP and LV dp/dt_max of the failing cat heart induced by pentobarbital significantly increased, CVP and LVEDP slightly decreased, while HR was without significant change. In addition, VBt was found to delay the onset of arrhythmia induced by strophanthin K and to increase the "safety margin" of the latter compound.Experiments with Langendorff's guinea pig heart showed that when VBt was gi-ven at 20 mg/min for 5 minutes, the normal heart rate was decreased and the contractile force was without significant change. The function of the failing heart induced by hypoxia was improved, and heart failure was prevented.VBt was shown to decrease the oxygen consumption of the rat myocardial tissue as measured by Warburg's technique.