Istaroxime is a new luso-inotropic compound selected for the treatment of acute heart failure syndromes, which reduces sodium-potassium adenosine triphosphatase (ATPase) activity and stimulates the sarcoplasmic calcium ATPase isoform 2 reuptake function. The aim of this study was to evaluate the safety profile of istaroxime. For this purpose, istaroxime was administered during a 24-hour infusion to conscious dogs with chronic heart failure and to genetically cardiomyopathic BIO TO.2 hamsters for 34 weeks orally. The parameters recorded were arrhythmic events and hemodynamic effects in dogs and mortality in hamsters. In dogs, istaroxime at 1, 3, and 4 μg/kg per min did not trigger arrhythmic events or magnify preexisting events. It increased left ventricular (LV) dP/dt_max (about 50% at 3 μg/kg per min) and LV − dP/dt_max (about 20% at 3 μg/kg per min) without changing heart rate, blood pressure, or double product. At 4 μg/kg per min, istaroxime increased dP/dt_max >100% but induced intense emesis in all animals. In cardiomyopathic hamsters, the dose of 30 mg/kg prolonged the survival rate to 32%. In conclusion, istaroxime seems to be a promising and safe new drug for improving cardiac performance in the failing heart.