Intrinsic features of the pumping process of a pneumatically driven ventricular assist device (VAD) and the effects of different types of pneumatic drivers upon its performance were investigated in vitro by analysing the pressure distributions within the device and the motions of the prosthetic valves. It was found that the stretching of the flexible, elastic diaphragm in both late systole and diastole initiates a pressure oscillation which directly affects the timing of the pumping process. The timing was also found to be dependent on the length and stiffness of the cannulae which link the VAD to the model circulation system. During the stretch-induced oscillation in late systole, the VAD housing experiences partial collapse due to fluid momentum effects, which tends to increase the effective stroke volume of the device, and reduce the amplitude of the pressure oscillation. Reducing the rising (falling) rate of driving pressures (dpd/dt) may not necessarily reduce the maximum rate of change of the blood chamber pressure (dpch/dtmax) but may upset the stability of the pumping process. This is because a minimum dpch/dtmax exists, which is determined by the stretch-induced oscillation. In order to minimize dpch/dtmax and to provide the device with a stable working condition, dpd/dt should match the dpch/dtmax.