A modular physical circuit for testing monoventricular and biventricular heart assist devices (HAD) is under development. The modules now available are assembled in an open-loop circuit and reproduce the function of the left ventricle and the systemic arterial tree. The left ventricle model reproduces Starling's law of the heart and can be easily controlled to modify other parameters such as contractility and timing (i.e. heart rate and systole/diastole ratio). This circuit, in connection with a left ventricular assist device (LVAD), can be used to evaluate the LVAD performance, its effect on the circulatory system and as a training system. This paper is devoted to a description of the circuit and of its interaction with a LVAD, which is analysed after the simulation of a low contractility pathology of the ventricle. Results obtained in these experiments are reported.