A lumped parameter model of the cardiovascular system (CVS) and its interaction with an implantable rotary blood pump (iRBP) is presented. The CVS model consists of the heart, the systemic and the pulmonary circulations. The pump model is made up of three differential equations, i.e. the motor equation, the torque equation and the hydraulic equation. Qualitative comparison with data from ex vivo porcine experiments suggests that the model is able to simulate different physiologically significant pumping states with varying pump speed set points. The combined CVS-iRBP model is suitable for use as a tool for investigating changes in the circulatory system parameters in the presence of the pump, and for testing control algorithms.