We studied the positive inotropic effect of the novel cardiotonic agent, vesnarinone on open chest and intact anaesthetized dogs hearts. The results indicate that vesnarinone has a dose-effect relationship on increasing the contractile force, LV dp/dt max and stroke volume, and it shortens the ejection time 21%. No effect on blood pressure and heart rate was noted. Vesnarione is more effective than amrinone in terms of positive inotropic effect.